Star Control 2 has a mild version of this in general, due to the universe going on without the player - indeed, there's a time limit to winning the game - combined with the time-and-fuel-consuming need to gather resources in order to complete the game at all and the fact that many star systems are not worth mining. At least it's mild enough that one can play some "probe" games before trying to take a serious stab at actually winning...
The free remake/port, The Ur-Quan Masters, suffers from a specialised form of this trope because clues given in the original PC version are missing. For example, because UQM uses the dialogue from the 3DO version, it's missing two key lines of dialogue from the PC version (one about the time limit, the other about where to find a particular race's homeworld).
Also, one optional but important quest asks you to track down a unique life form on an unknown planet orbiting an unknown star in a game universe of hundreds of stars, with only an obscure clue about the constellation to help you find it. The solution was much more obvious if you owned the original PC release because the game came with a printed map. It has to do with the shape of the constellation.
Note that there is actually a way to get the coordinates in-game: if you try to get Fwiffo to tell you the coordinates of the Spathi homeworld by threatening him, he will lie and send you to the planet where the creature can be found. However, the player has absolutely no reason to do so, since Fwiffo has already told you how to find his homeworld, so it's still a considerable Guide Dang It.
One of the best examples is thus: It's possible to unlock five items, three of which are among the best equippable parts in the game, one of which is a highly versatile melee weapon, and one of which is one of the most powerful melee weapons in the entire game. The opportunity to do comes on Mission 15, which is about a third/quarter way through the game, making this secret very much a Disc One Nuke. All you have to do is take out three enemies instead of running away like the mission objective says you should. Sound easy? The three enemies are end-game-level bosses, they do not have reduced stats, and you have a total of four units for the entire mission. It's possible, even fairly simple once you figure out the trick, but most players won't realize that when faced with three enemies capable of one-shotting anyone they choose and told to head for the hills.
On the other hand, it does make sense to reward players for beating a supposedly Hopeless Boss Fight. Suppose it depends on whether or not the player is Genre Savvy enough to consider the possibility. Whether or not the trick to wining is easy to deduce though...
The Trick is easy to figure out...if you played the previous game, and recognise that you're standing in the same location as the final battle of the first game, which has the same Geo Effects as last time.
This has been going on since Super Robot Wars 3 where it's actually possible to pass up Quess Paraya since she's only in one stage, on a certain square and only Amuro can find her.
Company of Heroes and the Dawn of War series leave mentioning many things out of the game, to varying degrees of obviousness. For Company of Heroes, there's the 'Elite Armour' for certain infantry and for certain infantry to gain - it makes them be harder to hit and take less damage from most bullets, but always be hit by sniper fire and take more damage from flames. One would have a hard time realizing what infantry have this and what it does without looking into the Meta Game. Retreating in Dawn of War 2 is similar - doing so makes units run away back to their base, gradually getting fast and taking less damage from ranged attacks, but taken extra from melee. There is no indication anywhere why your units run through tons of ranged fire and survive, but get torn a new arse from melee attacks while retreating. And many units not made for melee get torn apart from melee units almost as fast anyway, so you'd also have a hard time to figure it out without going to the Meta Game also.
Ogre Battle 64: Person of Lordly Caliber is full of this. First off, the end of the game is based on your reputation, which is based on a complicated system that the game never tells you about. Second, in order to get many of the special characters and items you have to talk to people or go to certain shops at incredibly specific times, such as midnight on the last day of the month. Also the player's guide simply doesn't tell you how to get a handful of the characters in the game, despite the fact that they are, in a few cases, prominent figures in it, thus making it a Guide Dang It for a guide.
And then there's the original SNES Ogre Battle... Where does one begin? You often only get once chance to recruit the important NPCs, and the criteria for doing so are hopelessly vague and rarely alluded to in the game itself. This is made worse by the fact that the criteria often involve you having recruited another NPC, creating a knock-on effect whereby it's possible to miss out on almost every important character should you visit the towns in the wrong order on LEVEL THREE, without any indication of what the right order actually is. Getting the best ending also involves collecting all the Zodiac Stones, which are all randomly hidden and require you to have already gotten two obscure, difficult to obtain items - which can be lost, permanently, should you mess up getting them - before you can even begin the search. You also need to maintain high Charisma, Reputation and Alignment stats, which are - as in Ogre Battle 64 - related a system that the game doesn't explain to you. Have fun!
The game does explain - in game, even - how to get and maintain a good reputation: liberate towns with High ALI, High CHA characters. What it doesn't particularly tell you is how to boost ALI and CHA: delivering the killing blow to an enemy of higher level, preferably one of lower ALI. It alludes to it a little bit, but otherwise, you're on your own. The Canopus/Gilbert thing is All There in the Manual, by the way, as are some of the other various bits and pieces of getting the Zodiac Stones. Otherwise, there's a great deal of potential Lost Forever stuff in that game, in general.
The SNES Ogre Battle also had a rather unusual version of this, to get the worst ending in the game. It required having very high reputation to get a specific item, then almost immediately lowering your reputation to nothing so that a certain character would offer to join you if you gave him the item.
Many of the games in the Fire Emblem series feature strange and rather arcane ways to recruit various people to your army.
Path of Radiance
To convince a certain character to rejoin your crew late in the game, you have to have one inconsequential character talk to him, then have the main hero of the story defeat him, especially because the series is noted for the fact that character death is permanent the vast, vast majority of the time.
Another good example is Ilyana. A lonely little thunder mage in the top left corner of the map, with no indicator, prior battle conversation, base conversation, or anything to hint at her arrival. You have to talk to her with Ike.
The only way to get one of the best units in the game is to send one of two very powerful characters to a specific space in the corner of a field way out of the way. How many people discovered that one without a guide?
A Motive Rant for the Big Bad s hidden in a similar way. Said rant is obtained by letting him attack Reyson, a unit who can not harm him, (only 6 characters can harm him, only 3 of whom you can have at once) and will die quickly and permanently (as he can't take hits from normal foes...), best part? If you know about this, you will have already have seen the dialog, meaning there is no reason for you to do it.
There is also a Knight Ring that allows the user to use leftover movement after an action(like Cavaliers and Paladins for instance) that can be obtained from Naesala on Chapter 19. You have to talk to him with either Janaff or Ulki, then let Naesala talk to Reyson. Oh, and you can't kill any of the Ravens scattered across the stage that could possibly attack.
The Bands in general(Fighter, Soldier, Sword, etc.) that can be obtained in the second playthrough. They influence equipped characters' growths very slightly depending on which one. For instance, the Fighter Band gives you +5% to HP and Strength. However, anyone who doesn't know this will sell each and every one for the 2000 gold they're worth.
The Ashera Staff can be obtained from Chapter 22 by not killing any of the Priests or Bishops in your very small, narrow path. Granted, Ike mentions the priests are not your enemy, and you should avoid them. The solution to fulfill that is to simply shove them out of your way, a mechanic you would possibly use on your own units as opposed to enemies.
Rexaura, the best light magic tome in the game is held by a Bishop on the final chapter. Get him to at least half health by the end of the turn, he'll use an elixir, unequip his weapon, and you can steal it off him. Again, who figured this out?
Unlocking the super-secret, spoileriffic ending sequences for Radiant Dawn involves a convoluted series of events throughout the course of the story (And a New Game+) that nobody would ever do without actually knowing what would come from it. This even includes bringing in a maxed out Support Bond, transferred from the previous game.
The desert chapters present in every game from 6 onward (that are already annoying due to the fact that you are fighting on sand, lowering your units' movements) have hidden items that can only be obtained by having units wait on a range of squares, with a random chance of finding them each turn. The 10th game has these hidden items in EVERY chapter; naturally, finding any of them requires a guide or simply stumbling upon them. The game also doesn't tell you that thief characters have a 100% chance of finding these hidden items, compared to everyone else's percentage chance based on a skill that caps at 30, and that would require serious grinding and luck at that point in the game.
Geneaology of the Holy War has the Hero Axe. You get it by sending Lex, equipped with an Iron Axe (When the much better Steel Axe is also available) to a certain off-path square. How the hell does anyone guess any of this?!
Sword of Seals has something along the lines of this for the recruitment of a certain Paladin, who is considered to be one of the best pre-promoted characters. You have to drop one of your defenseless characters (Either a bard or a dancer, both incapable of attacking) next to a swarming mob of enemies. (6 to 8 cavaliers around the paladin you're trying to promote) While the storylines of the characters hint the bard can accomplish this (the paladin being an Etrurian general, and the bard is the supposedly dead prince of Etruria, who had merely faked his death) you can only have either the bard or the dancer in your party, and there is nothing indicating the dancer is capable of recruiting the general.
Sword of Seals also had the path to Ilia vs. the path to Sacae, which is based on the levels of certain non-essential characters, as well as the slightly non-intuitive methods used to enter the Gaiden Chapters in which the legendary weapons are found. These range from the easy (Chapter 8x has no turn requirement, only that Lilina survives the chapter) to the arcane (Chapter 20x, the last chapter of the Ilia/Sacae split requires you to complete chapter 20 in 25 turns while recruiting the unit that can be obtained there and all earlier characters related to that character must also still be alive and recruited) to the absolutely infuriating (Chapter 16x requires an enemy unit that can't be recruited during the chapter to survive Chapter 16; said enemy will automatically join you at the start of 16x and must be used in said Gaiden Chapter.) And you must get all eight legendary weapons (and the Holy Maiden staff) and not have used up any of them (the staff, in particular, has only three uses), plus keep a certain unit alive, in order for the game to continue past Chapter 22.
Getting some of the bonus chapters in Blazing Sword can be incredibly frustrating.
How to get the special double-Gaiden chapter 19xx in Blazing Sword? Simple. First you must play through Lyn mode and get the bard Nils up to Level 7, a feat that is guaranteed impossible to happen if you aren't specifically planning for it. Then advance to Hector mode, unlock chapter 19x, and kill Kishuna, an enemy unit who does not attack back in any way. Doesn't sound too hard. Except when you realize that you only have one turn to take him down, he has 50HP (higher than anyone else you are likely to have encountered), and can dodge nearly every attack you throw at him. It's incredibly difficult to take him down without RNG abuse. Of course, most would agree that the story revealed in 19xx iscompletely and utterly worth it.
The Gaiden chapter "Genesis" (22x in Eliwood mode, 23x in Hector mode) has the requirement of needing to obtain 700 points of experience in the regular chapter, "Living Legend", as well as having had recruited the character Hawkeye. Many players are already using very high level units, probably promoted as the two chapters allowing arena abuse in 16x or 17x "The Port of Badon" and 20 or 21, "A New Resolve" are played through previously, making this obscure requirement that's key to unlocking the full story of Nergal and Kishuna quite difficult. The presence of Pent, who cannot be recruited during the chapter doesn't help, as he will absolutely tear through the entirety of the enemy force himself unless rescued by a flying unit, which is quite risky because there are archers everywhere and flying units are weak to bows.
Sacred Stones has secret shops, which otherwise rare sell promotion and stat-boosting items, in Chapter 16, "Ruled by Madness", and Chapter 14A/14B, "Queen of White Dunes"/"Father and Son" (only one of which is available, depending on the route chosen). They're unmarked on the battlefield or world map, no hint is given that they even exist, and they can't be accessed again until you've beaten the game. To make matters worse, the Member Card, which is needed to access them, is only held by Rennac. It's easy for him to escape without joining the party on the one mission where he can be recruited.
Thracia 776 has Chapter 12x. Specifically, there are a bunch of Dancers on the map, and if you let them escape and don't kill any of them, another will show up on turn 25 with a Knight Proof. You only have about three turns to steal it from her before she makes her escape. And then five turns after that, if you did not kill that Dancer, yet another one will appear, this one with a Warp staff. It's completely arbitrary and there is no way of knowing how this will happen. It doesn't help that the map has darkness that makes it impossible to see them if you're not close enough.
The chapter itself can be trivialized by warping over one of a few characters to talk to the boss, who then joins your party, causing all the other enemies to leave. Incidentally, this brings up another Guide Dang It; Lara, a Thief you get early on in the game, has a hidden class change that can only be obtained by having her talk to the boss. While it's obvious that she can talk to him (if you bring her for the mission), the class change bit is only vaguely hinted at.
In Fire Emblem Awakening, Prince Chrom must be married to either of his prospect five girlfriends by the end of the first arc. Many people have found themselves getting him hitched to a Female Avatar by complete accident. Or marrying him to said Female Avatar through their supports, which some decry for their... polemic writing and possible OOC behavior. Or missing out on hitching him to to Olivia, a woman who joins the cast at the very last stage of said first arc.
Even the ROM hacks have moments like this. Case in point, The Last Promise (a Blazing Sword hack with a totally original plot and cast of characters) has finding the Emblem Lance, which requires Corben to visit a small house in chapter 2. This is a curveball because anyone who's played a FE game on the Gameboy Advance would expect only bigger houses and villages to give items. And getting all the Emblem weapons will unlock a sidequest near the end of the game, but fortunately none of them are as obtuse to find as the lance.
Then in Chapter 5, there is a sidequest unlocked by finding a Talisman. You'd think the Talisman would be in a chest or in some enemy's inventory, but instead, it's hidden from view...in a locked room that, other than one mook, is totally empty. Players would probably just ignore it if they didn't know there was a hidden item there.
In Advance Wars, in the original game the Final Battle against the Big Bad Sturm consists of a 3-on-1 match. The 1st CO is always Andy, but the 2nd and 3rd are seemingly randomized between the other CO's encountered. That is, until you read an online guide and find out the formula for getting the CO's you want. The Blue Moon CO depends on who you used in that first fork in the road at the beginning of the game, Max Strikes; if you used Andy, Olaf shows up to help at the end, whereas if you used Max, it's Grit. The 3rd CO depends on what you did in Green Earth and Yellow Comet; if you completed the three Missions against Sonja (which had requirements to unlock by themselves), Kanbei is your 3rd helper. In the Green Earth Missions, using Andy for all of them nets you Drake in the third position, while using Sami for all of them gets you Eagle (as well as the Bonus Boss Mission Rivals), both of whom override Kanbei. Doing none of these things results in Sami in the third position. The map is winnable regardless of who your combination is, but certain players do better with certain CO's, and the CO's start with different and somtimes better stuff (for example, Kanbei begins with several tanks and other heavy units while Eagle starts with an air squadron).
In Command & Conquer Generals, the last scenario of the Chinese campaign gives the enemy a superweapon that will screw you over every five minutes. Turns out, it won't fire if you have less than $5000. But of course this fact isn't mentioned in your briefing or anything.
Even worse, $5000 is the exact amount needed to build your superweapon, which was just unlocked in the mission prior. This does not mean you can not ever use the superweapon, since if you rush up the tech tree and start building as many nukes as you possibly can, then you'll be fine as long as you do not have $5000 when then enemy's superweapon is ready to fire.
When a mission is completed in Final Fantasy Tactics Advance, you often get a item that doesn't go into your regular inventory, but can be taken along on a mission, and boosts your entire party's stats in some way. Some missions force you to bring along these items before you can even begin (for example, you'll need to bring along the black thread and magic cloth if your mission is to make a hat for a black mage). Unfortunately, you quickly get more of these items than you can hold, and, while some of them are never used to complete a mission, quite a few of them are used hundreds of missions after you've gotten them. If you delete one of the latter items, they're Lost Forever.
This is especially infuriating with the black thread item as you are able to get 3 of them over the course of the game. Since you may well have 2 or 3 of them in your inventory at the same time, you may think that it is one of the items you can get many multiples of and when you next reach maximum item capacity you might think that discarding an item you have multiple of would be safe. The problem is, each of the three black threads is required to complete three separate missions. And this is one of few items you can get multiples of, nevermind being one of even fewer items that you can only get limited multiples of. Argh!
Secret characters come with their race's Ultima attack mastered and join at above-average levels. To have a 1 in 5 chance of recruiting them, you must either complete a certain mission or complete any mission while voluntarily bringing a certain mission item, depending on their race. The characters or methods to recruit them aren't mentioned, and the mission items needed to recruit them have no other use. The Nu Mou character is Lost Forever if he doesn't offer to join after his non-repeatable quest is complete; the Viera character's quest is repeatable but she's Lost Forever if you flee any mission leading to her recruitment quest.
The sequel makes things a bit better by having such items part of the loot system which is also used for Item Crafting and never Lost Forever. However, there are other instances of Guide Dang It: For instance, a series of quests where you need to provide the right person for a non-combat job, with very vague hints provided which race/class can do it. The worst case is "Wanted:Caretaker" which can only be done by a Viera White Mage - all the others require a specific class/set of classes OR a specific race, but that one requires both.
Then again, the random nature of the loot can really screw you over. A Thief and an Enemy Scan will make things much easier for you. Except the game's not going to tell you what you're missing for a specific piece of loot, so knowing exactly what you can steal is pointless unless you get a guide to consult on what is used to make what.
There is, however, one item that is Lost Forever unless you look up a guide or are lucky. The quest to make the moogle bard Hurdy revolves around you taking two diffent loot items that only appear that once, and make him a Shining Lute. Except one of those items (the Strawood) is also needed to make a Brilliant Theorbo, another weapon for Hurdy... which can also complete the quest. If you make the Shining Lute, chances are you'll find another one later in the game if you want to complete all the sidequests... but no such thing happens for a Brilliant Theorbo. So you're stuck with the Bard and Beastmaster classes being unable to mastered; not really important for Hurdy (Soul Etude heals and restores status, but there's plenty of other choices), but the Beastmaster skills can be needed to get an Ahriman to use the Blue Magick you need to learn.
FFT's PSP remake had the "Agria's Birthday" sidequest. you have to have Agrias and Mustadio in your party, which most people will...but you also have to have kept the two generic knights that joined with Agrias, and THEN go to a certain city during a certain month...WHILE you have at least a half a million gil in your inventory. Screw this up and you miss out on a redonkulously powerful accessory.
Even in regular FFT for the PS1, character class progression is a complete Guide Dang It. Classes have levels in other classes as requirements and you are not told the requirements, so there's no way to know what they are except by luck or using a guide.
Good luck getting most of the secret characters without a guide. Worse, in order to get the one weapon that let's Cloud use his special abilities requires going to one specific random battle map, with a character that has the move-find item ability equipped, and sending that character to stand on one unidentified spot, to find the weapon.
Tactics Ogre does this. The Chaos Frame works in a very odd way that requires a guide to find out...or even explain. But the results are obvious. If Denim lets Kachua dies and becomes the new ruler of Valeria, the Chaos Frame affects the ending directly. Either he's assassinated or Valeria gets taken over eventually.
But even more obscure is if you want all four elemental Shamans. You have to know to go through the chaos or neutral route to even get two of the sisters by chapter four (And did you pick the chapter where you have to save Seleye, as opposed to her being absent from the battlefield? God help you) and you are put against one of them named Shelley as a low-physical-defense enemy in chapter 4. While you are told in-game that you don't have to kill Shelley, it's still easy to do so by accident, especially if you are overleveled. But even if you do reduce her to about 20 health and she runs away...wait where is she? You got a tantalizing hint that she'll be playable! Well you have to go to Baramus, have Olivia, then enter training, then make it stormy somehow, then exit training, and then an event plays where she's recruited. Of course, this part is never specified in the game, how on earth would you discover this by accident?
The game Vanguard Bandits has Multiple Endings based on pivotal decisive moments. One branch of endings was only possible to reach if Bastion was level 8 by the end of the third mission (in other words... the only one to fight in all the battles up to that point, pretty much). Less obscure was a path that showed up after finishing the game at least once, but it was still fairly easy to choose the wrong decision at this point and just continue on the normal branch. The game showed no real sign that these branches exist.
Even worse, if your team's morale is low, it's impossible to even beat the game without getting the bad ending where the hero ends up possessed by the Big Bad. The worst part about this Nonstandard Game Over (apart from being forced to kill your comrades) is that your only option once this happens is to restart the game from the beginning.
While Disgaea: Hour of Darkness lets you know that there are Multiple Endings, nowhere in the included material will they tell you what factors affect these endings. This could lead to a great deal of frustration when you finally check GameFAQs and realize that the one accidental ally kill you made (easier to do than it sounds, with no non-ending-related repercussions whatsoever) disqualifies you from getting the canon ending. Or for that matter, that having an obscenely high number of ally kills at certain points in the game can earn you an early bad ending. In the DS version, there is at least an indication that the game keeps track of ally kills, but no indication of why.
Makai Kingdom is a little more merciful. If you get a different ending, it does have the decency to tell you what you did to get it, so at least you can avoid that one.
Assuming of course it isn't a level you NEED to beat. Some endings refuse to let you start the game over until you beat it (Kill too many allies and you'll get one like this), and some refuse to let you continue if you lose, even if it gets an ending. Is this your first playthrough through the game? Better get grinding or suffer an unwinnable battle forever.
Having Laharl as your leader when you fight Baal controlling Zettas body gives you a bad ending. The guide dang it comes in the ending itself. Once you win the battle with anyone else as a leader, you permanently unlock Zetta and thus cannot play the level again, even with subsequent playthroughs.
Speaking of Disgaea, to find Etna's Journal in the PS2 version, you had to flip two switches there were no indication existed, then examine a random corner with no indication that there was anything special about it — although all you miss out on by not finding it is a different perspective on the story and a rare item towards the end of the game. The remakes each make it a bit easier to discover, along with making it unlock an alternate storyline once you complete the game — in the PSP version, a Prinny is added near the corner you have to examine, commenting that there's a draft. The DS version also has the Prinny, and further adds notification balloons when you're near something you can interact with, making it easier to stumble across the switches by chance.
The requirements for unlocking the Dark World maps in Disgaea 2 range from the simple things like not taking damage, to bizarre ones like spending 30 turns on a particular map, or defeating all of the enemies with tower attacks. You're not given even the slightest hint about what the requirement for each level is.
Good luck getting to the Land of Carnage in Disgaea 4 on your own. The Promotionhell Tickets and the X-Dimension were one thing, but you need to get a very specific set of ship parts in order to get there. Parts that can only be found by torturing specific monsters for specific locations.
The Variable Sword in Mega Man Battle Network looks like a regular Sword with 160 power. However, by inputting button combinations that the game gives only a few of the less useful ones over the ingame BBS, Variable Sword can change its shape. It still has only 160 power, but can hit 4 times against anything (and only in the 3rd game). Even Neo Variable Sword (added in 4), with 220 power (with one combination hitting twice), isn't great either, and even worse, Neo Variable Sword's button combinations (which again, are never told by the game to the player) are different from the regular Variable Sword.
The gutspunch family of chips can apparently be fired as a rocket rather than a punch with their own button combination—and by their own I mean "each one has its own". Considering that without this they were effectively a sword that knocked people backwards, only some of the combonations are given on the ingame BBS. More importantly, in all of the games finding boss rematches (and thus the mega-class chips) for non-allied bosses, because after beating them a "ghost" appears in a specific unmarked unhinted-at dead end in if you're lucky the general region you explored just before beating him, which does not appear on the map and is virtually always in a dead end meaning the only way to find them without a map is to systematically walk into every single dead end of every internet area blindly. And after that further rematches against further-powered-up bosses for for further-powered-up chips become random encounters (ugh) on a different map. Forget the "secret areas", mystery data and hidden jack-in points; it'd take a masochist just to find all the rematches without a guide. Some jobs also require you to go to rather nonsensical areas to complete them, like the memorable occasion of finding an escaped penguin hiding in your bathroom in the sixth game.
Mega Man Star Force has invisible ghost bosses as well, but in the second game they're visible until you beat them and they become random enemy encounters. But enough about that - the biggest Guide Dang It EVER occurs to a translation error: At one point a character says, "You can have a Recover150", but you don't get a Recover150. He actually WANTS a Recover150. Thanks, Crapcom.
Secret Chips. Most of the Mega Man Battle Network games had them (the first one didn't). In the sixth game, it's possible to get "Secret Complete" by having a friend get all of their exclusive chips, then trade Libraries. That's the easiest game to get secret chips in. The fifth game has this as well, but also has two chips that can only be obtained by linking up with a completely unrelated game (Boktai 2), trading the points obtained by doing so at the Boktai Trader (you get points in Boktai as well that can be traded for armor), and hoping you get the chips you want (which you need to trade 50 points at once to even have a CHANCE of getting). The Crossover itself is mentioned in the instructions for both games, though, so it's still fairly easy to figure out. Game 4 requires you to defeat a bunch of opponents in the Free Space, which can only be filled with the necessary type of opponents by linking up with another game. Even then, you only get the chip of the LAST opponent you face, which is determined pretty much at random, and each Navi has three levels of chip they can drop, depending on how strong or weak the game considers them to be (not that the criteria for "strength" or "weakness" is well-known). Even this can be discovered without too much difficulty, though. In addition, there's the Z-Saver [sic]. Unless you or a friend are fans of Mega Man Zero, you wouldn't know that Zero 3 has the chip. Worse, if you happened to buy it used, it's entirely possible to not have it in the cart anymore.
What you'll have the most difficulty finding are the Secret Chips in games 3 and 2 (though they weren't yet referred to as such). This required getting a certain number of completion stars, then battling with a friend on multiplayer, with a random chance of the victor drawing a Secret Chip from nowhere instead of getting a chip from their opponent. You could actually tilt the odds in your favor if you knew how to, and if you're looking up how to get the Secret Chips in the first place, you might as well look this up too.
There's one more "Super Secret Chip" in Mega Man Battle Network 2. To obtain it, you must first get 100% completion, including obtaining all 10 Secret Chips, then save and return to the title screen. With your 5 shiny, multi-colored completion stars, you then have to hover the cursor over New Game and input a button combination that turns the letters orange. If you start an orange New Game, you'll be playing in Hard Mode, where the enemies deal 1.5x the damage to you, and have 1.5x the health. To make sure you don't just trade for chips from the later part of the game, the multiplayer modes are disabled (which also prevents you from getting the Secret Chips if you somehow clear enough of the game). If you manage to get through this ridiculously hard game (less ridiculously so the farther through the game you get), you're awarded with a "Congratulations" screen and the Super Secret Chip known as Sanctuary...which appears in your NORMAL file...which you've already 100% completed...
Then there's the chips that CANNOT be obtained legitimately without going to a special event held way back when, God knows where. These included the four elemental Gospel chips, which could be used to form the Game Breaker Program Advance known as Dark Messiah (P.A. number 31/30 in the P.A. Library). This only-usable-by-cheating P.A. is then referenced in game 6 with its perfectly legal Dark Messiah NEO...
Mega Man Battle Network 3 is rather unkind to those without a guide. If you manage to clear the game, then obtain all the Standard-class chips (including the ridiculously hard-to-find Viruschips) and Mega-class chips (including those only obtainable by trading with the other version), you can then unlock battles with the Omega Navis (which drop V5 chips, which are Giga-class)...by inputting a certain button sequence on the title screen. You then have to actually find the Omega Navis, and actually fighting them requires beating up a few waves of Omega-level viruses first, all in a row, which will likely leave you weakened for the upcoming boss battle. Did I mention that one of the Program Advances (needed for the P.A. Complete star) requires using one of three of these V5 chips, alongside two other chips that are quite pathetic on their own?
Not to mention how several of the chips in that game could only be obtained by 1. defeating a virus, 2. with a Busting Rank of S, 3. in under 5 seconds, 4. while in a Custom Style (no using, say, HeatGuts Style), 5. without using the Mega Buster or any chips that freeze time. This is referred to as a "Special Custom Drop," and while most viruses just drop their usual chips in rare, hard-to-find codes, some viruses dropped completely different chips (BodyBurn becomes Burner, LavaCannon becomes Volcano). Worse yet, replace "under 5 seconds" with "under 20 seconds," and you've got the requirements to obtain any Navi's V4 chip. Couple this with how difficult Navi ghosts can be to find, especially in that game (which has special conditions for several of the ghosts, like having a specific program equipped, or being low on health), and you'll definitely be shouting "GUIDE DANG IT!" before long. Oh, and if your friend with the other version can't do well enough against the Navi exclusive to that version, you can kiss your 100% completion goodbye.
In the plotline for Mega Man Battle Network 3, one of the clues to find a magical McGuffin to complete the game is "one of many birds". Easy, in a game with a zoo area, and many birdlike enemies to battle. Unfortunately, the many birds refer to the paper cranes around a sick patient's hospital room. All references to which have been stripped out of the English version.
Worse still, there's a later sidequest where the translators flat out left one of the clues in Japanese. See the full quest here.
Mega Man Battle Network 6 has "rare" viruses that show up randomly in specific areas. For instance, RareBombCorn shows up in JudgeTree3. If a rare virus is defeated, you can then use that virus in a fun little virus battle mini-game. Just try and collect all the viruses without a guide.
Oh, and the rare viruses also drop rare chips (or rare chip codes) at their highest busting rank (ReflecMet * chips, for example). Since it's pretty difficult to beat most of them in the absurdly short time required (without specifically preparing for it) and there's no explicit information saying the drops are any different...
The first Shining Force has a unique item required to promote one of the character types hidden in a random spot in a castle. The only way one would legitimately find the thing would be to manually search (through a menu!) every tile in the game.
Shining Force 1 also brings you Musashi and Hanzou. To recruit Hanzou, you must search a bush at the gateway to Runefaust. Note that the bush is slightly different from the other bushes in the area. THIS IS NOT THE CASE in the Japanese version, in that version there is nothing different about the bush. Musashi can be acquired by reading a small sign in Prompt. Gong is somewhat out of the way but can be found by accident. Yogurt is in the same location but through a hidden doorway. Domingo is also a tough one to get, requiring you to find an egg in a suit of armor and then track down an incubator several chapters later.
But wait, there's more! In the Game Boy Advance remake, the locations of Hanzou and Musashi are changed. By the time you realize this—upon trying the recruitment methods mentioned above—you have already missed out on them. Only by reading a guide specifically made for this version ahead of time can you reasonably get them on your first run, especially if you played the original. Musashi in particular has a very tiny recruitment window; you must leave town to engage in a battle, then retreat. If you continue to the next battle, he's (arbitrarily) Lost Forever. Well, until New Game Plus brings you back around, anyway. Even though you can still access the spot where he would have been recruited.
Leveling mechanics can also be a bit obtuse. In Shining Force 1, you were best served by not promoting your casters until they had hit level cap due to the way in which spells are acquired. Promoting resets their levels, and while not crippling stats, the spells are learned at specific levels regardless of that character's class. But in Shining Force 2, the game just assumes all characters were promoted right away at level 20 after promotion, regardless of how many levels were tagged on before promotion, so NOT promoting your casters right away slows down the rate of spells gained instead.
Shining Force II actually has a part where you are supposed to manually "use" an item to insert a slab into a tree. While this sounds easy, and the game makes it clear that this is your objective, it's actually somewhat confusing due to controls. While the player mostly uses the Genesis "C" button for most of the game to open the inventory screen, pressing "C" against this tree only opens a dialogue box. You must use the other button with a similar function, the "A" button against the tree, ensuring that you open your inventory and not "talk" to the tree. At no other point in the game are you forced to differentiate between the two buttons.
You need to do this at least two other times, actually: to acquire the Arm of Golem and the Force Sword, but both of them take place after the initial use of the slab.
Shining Force III saw you play over three chapters, each a different game, with interchangeable characters. Items are traded between them allowing access to characters, bosses and enemies are recruitable depending on what you did to them in battle and then there is the whole finding hidden characters throughout the game; one of which is found by buying a random egg and hatching it much later in the game with a chicken that appears at some arbitrary moment in your HQ. Oh and some hidden characters you find will disappear only to re-appear in other chapters.
Later versions of Dwarf Fortress are just about unplayable without recourse to the game's extensive wiki.
Battalion Wars 2 - in the mission Enemies Undone, if you didn't bother with the Xylvanians (most likely because they can't doing anything to you once you jump to the HQ) but wiped out all of the other enemies, you still won't get 100% in Power because you missed 8 infantry. Tip: they're all Xylvanians. However, a search reveals only 7 Grunts—still one short of the 100% in Power. It seems you get the 8th one by blowing up the 3 digging machines by shooting the explosive canisters near them, something suggested in-game by Vlad responding to that by warning Frontier's commander that this helps invite Xylvania to retaliate one day with their full wrath. What makes this more fun is that in other missions, some enemies won't necessarily count for Power at all, but you have to destroy all of the enemies that do count for Power if you want 100% in it.
Turns out to involve a Luck-Based Mission. But there is a more brutal Guide Dang It, which even badly hits a non-completionist player, in the first Battalion Wars: the Y button's use in commanded units' AI. Units in Follow mode will be far from active in attacking as opposed to in Wait mode where they will actively attack enemy units. However, using the Y button to specify a location for (a) unit(s) to move to will have the unit(s) attack anything that they get near enough actively. The game never suggesting about this may be part of why X-Day is regarded as a Scrappy Level (directing the units to inside the Artillery's range would stop the units from being hammered and have them attack the infantry support), and this causes a massive difficulty gap for Road to Xylvania as well, due to the AA Vets otherwise refusing to actively attack the respawning fourGunships.
Front Mission 3 has two completely different story arcs. How do you choose between one or the other? You choose to either go or not go with a character to a location. This happens right after the games first mission. The kick? One arc leads to the bad ending, the other to the good, and I believe one is MUCH more difficult than the other.
Game Boy Wars 3 has a few medals as this. Granted, the game gives you no hints as to how to obtain any of them, but these are the ones that definitely fall into the trope:
The Excellence Prize - clear all 45 Campaign maps in 54 battles. For starters, this automatically qualifies it as a Lost Forever. This is actually intended to keep players from Level Grinding on early maps and having a bunch of promoted units to work with—not that it will help much on the harder levels. However, it's particularly bad because in order to unlock certain maps without repeating others, you must clear certain maps slowly. Which maps you'd have to clear quickly or slowly is its own Guide Dang It too.
The 2 Engineer Medals - you must build certain terrain a certain number of times. This terrain must count as man-made and strengthening properties does not count toward this.
The All Unit Medal - you know how bad a Guide Dang It this is when even the maintainer of Game Boy Wars Network hasn't obtained it and must only even know of its existence due to hacking on somebody's part. It is possibly done by building all 51 units due to cheat search results, but can't be sure with a Japan-only Revenue Enhancing Device being required for the mercenary units.
Possibly the Reaper Medal (deny White Moon from surrendering 10 times).
The Kingdoms expansion to Medieval 2: Total War contains one of these in the Teutonic campaign. If you play as Denmark, you can form the Union of Kalamar which gives you access to three unique units as well as control of a large chunk of the map. How do you get this? Capture five cities that are listed at the beginning of the game, then kill the Norwegian King without accidentally taking over Norway. Sounds easy? Sure, if you're familiar with the location of medieval cities in the Baltic Sea. The five cities are mentioned once and you aren't given their locations. Three of them are relatively easy to find (they're right next to your starting location), one is on a island, and the last is tucked away across the map next to one of your enemies starting cities. Simply assassinating the king typically doesn't work due to the increased security and he's smart enough to not leave his citadel of doom. So, what you effectively need to do is attack the citadel, kill the king, and then retreat in order to ensure you don't accidental destroy Norway.
The C64 game Castle of Terror was legendary for this. Every nook and cranny needed to be thoroughly examined to move forward in the game, and even then the crucial object you needed to pick up was often not actually mentioned. Unsurprising, given that the game's creator has confirmed that it is quite literally Unwinnable By Design.
The third-to-last mission of Lego Rock Raiders, Back to Basics, has Slimy Slugs respawn endless until you either complete or fail the level, which of course makes your mission of collecting forty-five energy crystal nigh-impossible. What the game doesn't tell you is that the slugs don't start spawning until you've collected about eleven crystals, which means all you have to do is disable the "collect crystals" priority before you get too many, wait until you find a large collection of crystals in one area, build a Tool Store next to them and turning the crystal collectiong back on.
In the whole game, Chief only tells you three times about the monsters in that mission, the other times leaving them to be a nasty surprise. Oh, and one of those three times is a blatant lie.
There actually is a Real Life example of Guide Dang It in The Game of Kings; many non-professional players are unaware of a maneuver called En Passant (Gratuitous French for "In Passing"), where if a player's pawn is in position to take an opponents pawn if it moves forward, and their opponent makes use of the pawn's ability to move two spaces on its first move, the pawn positioned to capture may move as if the defensive pawn had only gone forward one space, taking the opposing pawn and more than likely giving it a straight shot to the last row. It's in almost every manual and guide out there, but no casual players ever really bother to read those, or if they do they soon forget about it.
New players to Sword of the Stars may be confused by the RandomisedTech Tree. It's almost impossible to have all the random techs available for research in any one game, meaning that one may not figure out all the routes to a certain tech or all the techs that branch off from one until many games' worth of experience (or consulting the wiki) later. The aversion of Interface Spoiler only makes it worse by preventing you from seeing the links.
In the DOS turn-based strategy game Master of Orion, occasionally you'll come across a random quiz where you have to identify various ship designs by name. if you get the questions right, good things will happen, like one of your planets suddenly becoming a Rich planet. But if you get the questions wrong three times in succession, you are TOAST. What makes this a Guide Dang It? Well, the answers to the quizzes can only be found in the original instruction booklet or in an online search for the Ship Guide. Otherwise, good luck trying to guess your way out of it.
There is actually a pattern to it but figuring it out still requires a lot of trial and error. Each faction color has it own unique ship designs. For purposes of the quiz each color is associated with a race and each race has appropriate names for their ships. For instance the bird aliens have ships named "Falcon" or "Warbird". Since the quiz is multiple choice a person who knows the pattern stands a chance of answering correctly within three tries. Keeping track of the correct and incorrect answers is almost required though.
Agarest Senki as a whole is just insane to complete without a guide. Good luck finding the right answers to properly raise the affections of the three heroines of the generation you are on without looking at the wiki. Or better yet, try unlocking the True Ending without a guide. It will absolutely tear your hair off.
Unlocking C Gs is also a hard deal in this game because you have to be in a specific karma level, and you also need to have your heroine in a specific affection level just to even get those C Gs.
Recruting the fifth generation characters as a whole are a Guide Dang It by themselves.
Recruiting one character who by all rights betrayed you is pretty damn hard if you didn't know that you're not supposed to kill him but instead save him. Nowhere does the conversation or the game tell you to not kill Vashtor. You didn't get him? Say goodbye to your True Ending.
Having Dyshana in your party (not as a tag along member) requires you to be in Neutral at the start of the fifth generation. That means you need to end the fourth generation at the neutral phase. Nowhere does the game tell you that you need to be in that karma level to even recruit her.
Recruiting Beatrice in your party requires you to save Nastassja back in the third generation. Although this one is easier than the rest, EXCEPT this is also the generation that has That One Boss so you may end up level grinding and not knowing that you're not supposed to level grind on points.
Getting Murmina in your party has you go to take the longest route of the three routes you are presented. There are three routes to choose from and you need to have less than 25 turns to reach her. Take more than that and she dies from her injuries.
Valkyria Chronicles has unlocking the Optional Party Members. While Musaad start a New Game+ and Audry get 10 meadals could be reasonably stumbled upon without a guide, Lynn and Emile both require you to unlock a specific character's hidden potential, then let them be KO'd (thankfully not killed, you are allowed to rescue them with the medic. In case you're wondering, the characters are Karl and Oscar respectively.) and Knute requires you to enter the command room with 1,000,000 in cash on hand. All these conditions make perfect sense once you find out the unlockee's personality, Lynn is Karl's lover, Emile is Oscar's brother, and Knute is a Miser Advisor) which you won't know that until after you unlock them.
The two characters in question also consist of one of the worst members for one class and a squad member only mediocre at their actual role, but if you know all of their potentials in advance, they are potentially useful for a different role their class is normally weak at (so further example of Guide Dang It). Without consulting a guide, it's fairly unlikely one of them will ever be taken and the other might get taken, but overshadowed by other members of the class more adept at their actual role.
StarCraft, has a small (but nasty) case of this in the second mission of Zerg campaign. You are supposed to move the cocoon to destination, but at no place is it mentioned how to move it. You have to pick it up with the drone, but nobody tells you.
The objective of the Protoss Civil War mission "Return to Aiur" is "Destroy the heart of the Conclave". Unlike all other missions with similar objectives, this doesn't mean "destroy their entire base", you just need to destroy their Nexus to win.
Completing The Reckoning in Brood War in under 25 minutes unlocks a bonus level. The only hint that you're missing something is Zeratul's disappearance from the rest of the campaign. This is a rare case of Guide Dang It interfering with the story, as the bonus level provides the backdrop for Zeratul's story in Starcraft II.
Unlocking the secret mission in StarCraft 2's Terran campaign requires you to destroy an unmarked building in the mission "Media Blitz". Nothing in the game suggests that there's a secret mission, or that there's something hidden in "Media Blitz".
The original Magic: The Gathering: Duels of the Planeswalkers has a puzzle that can only be solved by exploiting a bug that makes the A.I. Too Stupid To Live. The computer tends to use creature removal as soon as it has a target, even if there's no strategic reason to do so. You have to count on the A.I. using its Royal Assassin to kill a creature you've tapped for an ability (which accomplishes nothing) so it can't kill your biggest attacker during combat (which it needs to do to stay alive). You have no reason to think that it would react in this way unless you were aware of the bug.