Final Fantasy I's NES version almost manages to be this in its entirety. While the game's manual helpfully lists the effects of all spells and equipment (probably due to limitations in the game's text display as everything is truncated and comes with no further information than odd names like LOK2note Focara, XFERnote Dispel, Masmune and ProCapenote Protect Cape) if you got the game without the manual you wouldn't have any of this, and all those vaguely named items and spells may well have had you checking a guide. Similarly, the manual also contains a walkthrough of about half the game, probably circumventing the sloppy translation that otherwise makes certain early goals too vague. Again, no manual? Hope you had that one guide by Nintendo Power to tell you to go to the Marsh Cave and get the Crown and take the Crown to the Dark Elf in the Western Castle and so on because that NPC dialogue might not be so helpful! Thankfully everything was much clearer when Final Fantasy Origins and the many, many subsequent remakes came out with a better translation and interface, making everything coherent so the player could figure it out on their own.
Of course, if you lived in Europe and the Origins re-release was the first version you officially played, none of that was an issue. And they used to say PAL gamers always got screwed...
In Final Fantasy II, the game was good about telling you what needed to be done, but said very little about where it was located on the wide-open map. Going even slightly off the right path could send you into a Beef Gate that wiped out your whole party.
In the Final Fantasy III remake for the Nintendo DS, the only way to unlock the Onion Knight class in the game is to use the game's letter sending system to send a certain number of letters to another player over wifi. Not only is this never mentioned in the game, Square Enix seems not to have realized it might not even be possible for some people. Don't have easy access to a wifi hotspot? Don't have any friends who also have this game? TOO BAD. And with the shutdown of wifi connection for DS and Wii in early 2014, this is now impossible.
The iPhone version thankfully (and oddly, since it could have been recreated with Open Feint or something) removes the "mail your friends" requirement and simply allows you to get the mognet mails and sidequests by advancing the game and Talking to key NPCs.
The DS remake of Final Fantasy IV has, among other things, an "Augment" system wherein you can teach characters certain helpful abilities. These items are one-of-a-kind, and said character will know them permanently. The catch is, if you teach Augments to temporary party members, you are rewarded with more (better) Augments. This little fact is nowhere to be found in the manual, or in-game. What fun.
Final Fantasy IV The After Years provides the player with a way to avoid the Player Punch where Calca and Brina must be scrapped for parts, requiring them to get three items, left completely unmentioned by the game. One of the items is in an obvious jar the player is unlikely to miss. Fair enough. The other two, however? Random drops from a monster that only appears in one out-of-the-way room during one specific lunar cycle (which is the worst lunar cycle for a party with a black mage and no white mages, as the chapter in question is), and the drop rate is absolutely inexcusably low in this game. And you need two different items from this. Yet any player who knows about this will do it, because who could really allow Calca and Brina to die?
In all fairness, Calca and Brina are kinda useless after this chapter.
You can also save Golbez during his Taking the BulletHeroic Sacrifice for Cecil. Of course, you have to have a very specific party. Golbez, Ceodore, Cecil, and Rosa; in other words, Cecil's (and by extension Golbez's) family.
Bands. More specifically, searching for them. Some of them are unlocked with plot, others you can unlock anytime if you have required characters and input correct commands. Most of them make sense (friends/relatives usually get some), but then there are those ones that don't make any sense. After all, how could you expect Yang and Gekkou to actually have a Band, even though they didn't even as much as talk to each other? But the worst of them all is Call Me Queen. To perform it, you need to have four specific (female) party members. The catch? They all need to be equipped with whips. And one of them ( Leonora) can equip literally ONE whip in the whole game - the one that could only be bought WAAAY BACK in the chapter where she first appears, meaning, at the point where you can try to perform this Band it's already Lost Forever. And you probably wouldn't even KNOW it's THERE, because the shop that sells the whip in question can only be accessed by paying 50000 gil to get to an "exclusive" area of the chapter.
Final Fantasy V has the !Catch/!Release commands, which let a character catch a monster and then release it in a later battle. You're on your own when it comes to figuring out what effects a monster will have when released, and which monsters are worth catching.
Final Fantasy VI has a "Cursed Shield" that inflicts nearly every status ailment in the game on the wearer. But if they survive 256 battles wearing the shield, it becomes uncursed and is now the best shield in the game, and wearing it is one of the few ways to learn Ultima.
You can also bet the shield at the Colosseum for a "Cursed Ring". Despite what logic tells you, the ring doesn't uncurse, and is plain useless (except for teaching a spell, but it's not even the only thing that does that.)
Speaking of the Colosseum, that's another Guide Dang It! The betting list is a total mystery, you have no idea what you can win by betting what. A lot of rare items can be won here, but they in turn require long chains of complex betting no one could figure out alone.
Finding Gogo requires the player to let an enemy monster kill their party members one by one on Triangle Island in the World of Ruin so they can find him inside its stomach. In the World of Balance, this same location was home to the Nigh InvulnerableBoss in Mook Clothing Intangir, and nothing else.
Getting Shadow to be able to join your party in the second half of the game requires you- in a major dungeon around the midpoint- to intentionally stand around while a timer ticks down towards an instant game over, with escape literally a step away, in order to give him time to catch up, which he only manages to do with 5 seconds left on the clock. To be fair, the game does give you one hint about this... if, upon reaching the exit, you choose not to escape to safety, then immediately try again, you will get the option "Gotta wait for Shadow...".
And then you actually have to recruit him, which is itself rather difficult. Finding him is easy enough, but he turns up wounded, stops in Thamasa to rest, and then disappears again. You have to find a specific item in the same cave you find Shadow in, then, after he disappears from Thamasa, you fly to the Coliseum and bet said item. Win the fight, and he will join you.
Gau's "Rage" skill. How it works (which is explained by the game, although not very well), what the best rages are, what each rage will do...very little is immediately obvious. (A chipmunk can command the earth to swallow monsters whole? Alright. A stray cat has one of the most useful physical moves? Okay... The mighty Intangir has a penchant for suicide? Huh. A rhinoceros has the power of automatic resurrection...wait, what?) Experimentation and frequent visits to the Veldt will turn Gau into a Disc One Nuke machine.
In some dungeons early on, you can "upgrade" unopened treasure chests as you progress through the game. The game does not hint at this at all. Opened an innocent treasure chest early on in the game in the Narshe mines? Whoops, that upgrade's Lost Forever.
The skill "Chocobuckle" in Final Fantasy VII. To get 100% Completion of enemy skills, you needed to feed a wild Chocobo a particular green and then reduce it to 1 hit point. This was typically done using the Useless Useful Spell L4 Suicide. Needless to say there was no way to guess this in game, while the occasional player got it by pure chance and puzzled everyone else.
Barrett's ultimate weapon is in a chest that only appears if Barrett is in the party at that part of the game, and there is no indication from the game of this. Furthermore, you cannot return to the area where the chest is, so if you miss it, the weapon is lost forever.
To last any longer than 20 minutes against Emerald WEAPON, you need the Underwater Materia. But to get the Materia in the first place, you need to trade a specific item to a specific NPC. This item must be Morphed off an enemy that is in a location that you're not very likely to revisit(Underwater Reactor/Sub Dock), as well, said enemy has a habit of removing characters from battle like the Midgar Zolom. And, of course, said NPC is in the early game, long after he tells you he is looking for rare items. Then the second Guide Dang It comes when you actually fight it, and it deals 9999 damage for you having too many Materia equipped.
Obtaining Aerith's final limit move is extremely time consuming, and damn near impossible without a guide, as you first have to find a man living in a cave, who can only be gotten to if you have the buggy, or any vehicle/chocobo capable of crossing shallow rivers. The problem is that the game doesn't tell you that you can just drive the buggy into Costa Del Sol to use it on the other continent, and the man's cave is exceptionally well hidden. When you speak to him, he merely tells you how many times you've ran from battle. This seems extremely useless, but interesting, unless you know that if the last two digits of this number are the same (e.g. 44, 133, or 11), then he will give you an item. This is never hinted to in game. Sometimes he just gives you a bolt ring, and sometimes he gives you a piece of Mythril. You then have to take this piece of mythril to a weapon maker near Gongaga, and redeem it to get one of two items. One of these two items is Aerith's final limit, and the other is a piece of armor. This can only be done once, and the game gives no hint as to which one it is. Oh, and hope you manage to do this on the first disc only, since Aerith dies at the end of disc one, making this information useless past this point.
Even the guide doesn't tell you about the buggy trick, which is about the only way to get it while Aerith is still alive.
It is unlikely to encounter all the Turtle's Paradise flyers, especially on a first run through the game, and especially considering one is very well hidden on the ground floor of the Shinra tower (it's one of two signs on the same billboard, which the player will likely then ignore if they read the wrong one first) and as such is quite easy to lose forever. The sixth and final flyer also becomes permanently inaccessible if the player never visits the location they're supposed to find it in before disc 3, though it's far less likely the player will miss that on the first run... no, they'll probably miss it on subsequent runs instead.
Getting the PuPu card in Final Fantasy VIII. Hints about some parts of the process are given in the game, but not all of it, and these hints are fairly obscure themselves. The player has to fight random battles at several small, nondescript, arbitrary patches of the world map in order to see a UFO each time. They then have to go to another arbitrary, unexceptional, and inaccessible area of land in order to encounter PuPu. Once they've done so they have to feed him five of a certain item. The only way to have five of this item is to have synthesized them ahead of time, by using another fairly obscure game mechanic. If the party misses the chance to feed PuPu five of the item, or they kill him, the card is Lost Forever.
You can use the Card skill on him, but that would require knowing ahead of time that the point of the sidequest is to obtain his card.
Continuing on cards, the (in)famous Queen of Cards quest. Every time you beat the Queen of Cards in a game, she moves to another location. When she goes to Dollet you have the option of losing one of your rare cards to her so that she can make another rare card from it. Tedious enough enough right? The Guide dang-it part comes once these other cards are made, the game gives absolutely NO HINT as to where these cards are located, and some of them are so insanely random (would you really expect some random guy in Deling City to get the rare card) that its all but impossible to find the new cards without the guide to tell you who has them.
You can do this quest the easy, but even more Guide-dang-tastic way. Find a particular unmarked spot on the map on Disc 4, the Queen will be there with all her rare cards in her inventory. There are also the Card Club members on Ragnarok during Disc 4, for extra lost card recovery.
All of the enemies in the game have incredibly rare items to drop and steal needed for weapon refinement, but the enemy drops and steals change depending on the level of the enemy, making discovering what drops what by trial and error next to impossible.
In a Laguna flashback on Disk 1, the player can fiddle with explosives, find and lose a rusty key, and fiddle with some hatches. On Disk 3, if the player did all of the above correctly, three doors will appear in a particular dungeon, otherwise they'll be closed and the valuable items behind them are Lost Forever.
Many of the Guardian Forces are very difficult to find, and a good number of them can easily be Lost Forever. Most notable is the Tonberry GF, which can only be obtained by killing the Tonberry King, who will only appear if you kill about 20 Tonberries in a row. There is absolutely no clue about this anywhere in the game.
And good luck getting Doom Train, which requires gathering six each of three different items. There are hints to getting him available, in the form of occult magazines, but not only are they so vague to the point of being nearly useless, but you'll need a guide to find them as well!
Receiving the most powerful weapon in Final Fantasy IX, the Excalibur 2, requires that one must complete the game within the very difficult time limit of 12 hours. Not only is this information not given to the player at any point in the game, but the location of the weapon is just as difficult to find. What's worse, once the time limit has been reached, the weapon is Lost Forever. The guide also doesn't tell you that there is a technique that allows you to skip the cutscenes completely, which certainly makes the time limit easier to cope with. (In fairness, the makers of the guide may not have realised this.)
Hell, getting the first Excalibur is no picnic either. There's nothing at all that even hints at how you're supposed to find the MacGuffin a certain NPC wants (you have to buy four otherMacGuffins and then sell them around town for it to even appear), and then you have to bid a king's ransom at the Auction House to buy it. Not only not fun, but it's not even his best sword! (That would be, not counting the above mentioned Excalibur 2, the sword Ragnarok, though Excalibur is the only sword that can teach Steiner one of his most powerful skills...except the one Ragnarok teaches is even more powerful. Go figure.)
The most absolutely frustrating thing in Final Fantasy IX, though, is that you're never told at any time what items you need to synthesize the very best weapons and armor in Disks 3 and 4. On your first playthrough, be prepared to pull out some of your hair in frustration as you realize that, in order to forge the Grand Armor, you needed to keep those Mythril Swords and suits of Mythril Armor you got all the way back in Disk 2. Oh, and the Mythril Swords become Lost Forevers after you leave Treno for the first time. That's just one example, mind you...
Theoretically speaking, one could go to Esto Gaza before the plot demands it and buy their lion's share then. However, the above still applies, as many don't really think about Esto Gaza during that time.
The Synthesis system in general will turn you into a hoarder with your items. Usually, most players will sell old equipment that they no longer use, only to kick themselves when they see that a powerful item to create needs some old items that you used to have and most likely can't get another one. Players who are playing the game more than once (or looked up an FAQ online) can avoid this trope, but many first time players were not pleased to discover that it is better to save your old stuff so you can use them later to create better items.
There's one scene in which you can perform a certain action, and it does nothing. You have to do it thirteen times in succession to reveal one of the secret items. Needless to say, there are no hints for this.
And a mention really must be made of how Squaresoft gave up on even disguising the Revenue Enhancing Devices by making the Official Strategy Guide fairly bare-bones. "For more information, visit our website and enter this code!—" No, seriously. They had the sense to put enough in the guide that you could muddle through the game, but for a while they had the first guide that was liable to provide Guide Dang Its of its own.
The final twist in the knife has to be the Nero Family side quest. It involves proceeding forward into the final dungeon, then leaving immediately after each cutscene and boss to backtrack to Tantalus' hideout in Lindblum. You have to do this nine times, and at the end, you get a Protect Ring. If you fail to backtrack too many times, then it will be Lost Forever. This quest was so obscure, that it was discovered and recorded for the first time in 2013, nearly thirteen years after the game came out!
Also the ultimate weapons and the ridiculous hoops we had to jump through. The biggest example would probably be having to dodge 200 lightning bolts in a row. There is no in-game hint about how many you have to dodge, or even that there will be a reward this valuable; without a guide, many players would probably stop around 50-60 and assume the item they were given was the final prize. Not to mention it's extremely hard, so even the Genre Savvy players who somehow guessed there was a better reward would stop before they actually reached 200 dodged bolts.
The Chocobo race also deserves mentioning. You are required to finish with a time under 0:0:0. No sane gamer alive could reach such a conclusion without a guide.
And to make things even worse, even if you KNOW you have to finish with such a low time, the random factor of the items that lower your time and the utterly ATROCIOUS controls basically make it a Luck-Based Mission that few sane gamers alive would even persist in trying to complete under those conditions.
Getting the Celestial Mirror to unlock some of those ultimate weapons is, itself, pretty silly. First you have to win a chocobo race at Remiem Temple. Once you've managed that, you have to take the Cloudy Mirror to the Macalania Woods and reunite a man with his wife and child. When you do so, if you talk to the man twice, he'll ask you to find his kid, who has wandered off. If you go up a path that you may not instantly realise is even a path rather than awesome scenery, which looks like it's a beam of light and thus something you wouldn't immediately suspect you could walk on, and which isn't visible on the minimap, and then go up a fork that was until that momentNPC Roadblocked. Now, bear in mind, this item has no use at all except as a key to boxes containing ultimate weapons.
Valefor's second Overdrive, Energy Blast. Did you know it exists? If you haven't either read a guide or encountered Dark Valefor, you probably didn't. As for actually getting it, if you didn't talk to a dog in Besaid before you left, you won't have a chance to get it until you can throw down successfully with Dark Valefor. Bear in mind that the Dark Aeons are only exempt from being a collection of That One Bosses on a technicality.
At least the Dark Aeons only appear in the International, PAL and HD remake versions.
To get 100% Completion in Final Fantasy X2 one has to take a detour from chasing a villain in order to talk to someone hidden in a Moogle costume, early in the game. The game is riddled with one-time, easily missable scenes like this, and despite the fact you get fully healed from touching a save point, you have to use the bed in the airship at least once a chapter. And that isn't even the worst part. The game allows you to skip cutscenes, but what it doesn't tell you is that skipped cutscenes doesn't count towards the 100% Completion.
There's one bit even worse than that. At one point you can have a long sit down for a Maechen Period from the original Maechen himself. Periodically, you'll get a text box where you can either interrupt him to leave, or urge him to continue his story. But what you're supposed to do for this to count toward completion is NEITHER, and let him just keep rambling without you pressing a single button on your controller. If Maechen wasn't voiced by Dwight Schultz, this would be nearly as tedious and unbearable as the legendary hot-springs webcam sequence.
Speaking of, the Comm Sphere sequences in Chapter 4 itself. Unless a guide is right by your side, you'd never know that a few scenes only appear after looking through the camera for a while, after other scenes that have nothing going on, and the whole Mi'ihen Highroad clusterfuck, of which, a number of potential targets don't give any completion points upon the reveal.
And let's not forget that to get the best ending, you have to wait until Yuna says "I'm all alone..." then press X to hear a whistle. Then after that, you just keep pressing X until she runs out of the Farplane. The second part is much worse, as you need to press X at a specific point during what is essentially the last cutscene of the game, in order to get the Perfect Ending.
And how, when at that point of the game, the Youth League and New Yevon are portrayed as being both basically good except for their conflict with each other, If you choose to help New Yevon and not the Youth League (instead of vice versa), you can't get 100% completion on that playthrough.
Likely just an accident. Complete % carry over in a new playthrough, so it's likely that players were only intended to get 100% after playing once with each route. Also, % from all scenes add up to more than 100%, so you can afford to miss one or two and still get 100%, if you play both routes. The Youth league route giving 100% in one playthrough if you get absolutely every single possible scene (really annoying to keep track of, since some give less than 1%) might be a mistake. But it's absolutely impossible without a guide, and still damn hard with one.
Want the Mascot Dressphere? Get episode complete on every area. How do you do that? Good luck finding that out, because it's not mentioned anywhere in the game. It's a game spanning quest, because there are certain objectives to fulfill in every chapter.
Want to fully power up Rikku's Special Dressphere? You'll need access to the Chocobo Dungeon. How do you unlock it? Good luck finding that out, because it's not mentioned anywhere in the game.
Want to fully power up Pain's Special Dressphere? Are you noticing a pattern yet?
Let's not even get into the stuff that would be way too obtuse to find if the game files weren't so heavily picked apart for information...
For example, to obtain the useful unique armor for the Scholar class, you are told to find some random object. The name at best suggests a single zone to look in. Now, amongst the items you need to get, one only appears during certain weather, transporting between many spots in the zone each time it rains. Another only appears at certain times of the day. In addition, these objects (And the vast majority of quest items not dropped from monsters), do not actually appear on the map. Rather you must mash the tab button until a blank point on the ground gets highlighted (Labeled as ???, just in case you might have thought to write down what you found and where you found it for later). Even with guides telling you where and when to look, it can still take hours.
And yet we still haven't talked about the gardening yet. It sounds easy: buy a flower pot and get some seeds, then put the pot in your Mog House and plant the seeds. What they don't tell you is that the seeds will not only have different yields, but also give you different crops depending on: what day of the week you planted them on, what elemental energy is flowing in your house, what crystal, if any, you feed the plant when you have the option to, how long you leave it planted, how many times you examined it per day when it was growing, and the phases of the in-game moon. Squenix apparently loves having you figure crap out by yourself because time it takes you to figure it out means more subscription money for them. Those magnificent bastards...
Crafting. Just... Crafting. While yes, the NPCs in the guilds flat out tell you SOME recipes for random items, they don't tell you all of them. Especially, ya know... The useful ones.
In Final Fantasy XII, obtaining the Zodiac Spear requires not opening four specific unmarked chests which are not mentioned anywhere in the game. This would be a prime example. Granted, if you got greedy and opened the "unlucky" chests, there is another chest that can cough it up, roughly .1% of the time (that is to say, 1/1000 chance)... but the said existence of that chest is in and of itself an example of Guide Dang It.
There is also the case of the Bazaar system, by which selling loot is the only way to unlock some of the high end items. The loot items needed for these are not only very difficult to acquire, but are also used to unlock other, more easily unlocked, items. And once an item of loot has unlocked one item for sale, it must be acquired all over again and sold once more.
The crowning example of this is, of course, the Tournesol, which requires you to get multiple copies of three different items. Let's take Empyreal Souls, for example, of which you need three. You'll need six Wargod's Bands, which are Rare drops from high-level monsters, three Soul Powders, which are Rare Steals off of Rare Game, and three High Arcana (which adds another level to the madness, since you need to purchase a Canopic Jar to get those which takes three OTHER items). You must then sell one High Arcana, Soul Powder, and two Wargod's Bands, buy an Empyreal Soul, and then restart the process. If you sell too many at once you'll have to go find more. You must do this again with two other items, each comprised of three other rare drops. To access some of those you need to purchase Monographs. The game doesn't tell you that you need the Monographs to get particular items you need, or how to access the Monographs you need, or which Monographs you need. All of this will cost you several thousand gil, and hours upon hours of time. And none of it is hinted at anywhere.
Don't forget that if you want to get all eighty rare game, you have to either be the luckiest gamer in the world or have a guide by your side. Many of these monsters just have a high-percentage random chance to show up, but many more have an inexplicable list of criteria that need to be filled before they might rear their ugly heads. Some appear for only a ten-minute window once an hour. Some require you to chain-kill a certain number of a certain kind of monster. Some have time limits on top of this criteria. Some require you not to kill any monsters at all. Some appear in tiny, tiny areas, again, randomly. Some have chances of appearance at just 5%. Some only appear if you sit around doing nothing for five minutes or more. Sure, the average gamer stumbles across at least a few of them by accident across the game, but all eighty? Forget it.
The worst part about this is that the guide only tells you how to spawn about half of them, it tells you the location for the rare game but not the spawn conditions, have fun experimenting with generic conditions to get them thing to spawn but even that won't always work because some of these monsters have completely unique spawn conditions that are only hinted at in the bestiary, which only appears after you kill them. Have fun looking through online guides to find out how to get 100% completion
The game also riddles players with the Limit Break of the Espers. While a good portion of the creatures will use their last attack when time is about to expire, low on HP, or the summoner has low HP. However, some of the other Espers will never use their final attack unless certain conditions are met, such as casting Immobilize on the Esper, having the summoner AND the Esper with low HP, or casting Petrify on the Esper! There is NOTHING in the game that hints at these conditions.
The weapon upgrade system is needlessly complex and not explained well. The game tells you about the XP multiplier, but it doesn't explain how it raises and lowers, and it's not really easy to figure out without wasting a ton of money and components.
"This is a secret achievement. Unlock it to find out more about it." Or read a walkthrough if you want to achieve it, instead of blindly stumbling across it.
The ATB Refresh technique is not mentioned anywhere in the game. If you have two identical paradigms in your deck (e.g. two Relentless Assaults), switching between them will cause the ATB gauge to fill up immediately under certain circumstances. This also goes unmentioned in in XIII-2.
Final Fantasy XIII-2: One word - fragments. Collecting all 160 requires several Guide Dang Its in itself:
Completing all Brain Blast and Captain Cryptic quizzes in Academia 4XX AF. Several Brain Blast questions either have no hints in-game to their answer, or are based on pure luck (heads or tails?). Captain Cryptic's questions are worse - and first, you have to find him. He can be found in 11 different locations across town... and he's invisible.
Completing the bestiary will get you one fragment. Probably 90% of the monsters will be encountered as you play the game, level up, and go after the rest of the fragments, but some are either incredibly rare, hidden in unintuitive locations, or just maliciously placed.
The Proto-fal'Cie Adam has three forms, each of which get their own bestiary entry. However, the third form is skippable if you pick the right answer during the Live Trigger segment.
The Giant Cactuar. No, not the Gigantuar. If you leave a Cactuar alive long enough, he'll eventually use Cactus Dance and transform into a Giant Cactuar, which gets its own bestiary entry.
Raspatil, a hideously powerful Undying Cie'th, is an extremely rare enemy that spawns only in a small area in Oerba 400 AF.
Tezcatlipoca, an albino Woodwraith, is probably the most dickheaded hidden monster. It appears in one place, on a platform at the end of The Very Definitely Final Dungeon, with the Battlemania fragment skill turned on. And, if you fight Xolotl and Miquitzli, the monsters that show up there normally, you have to reset the entire dungeon and play through it again.
The Lucky Coin fragment which is nothing more than a Luck-Based Mission. It's a fragment you get when you obtain a total of 10.000 coins from the casino and there is no real way you can influence the slot machine to go your way. Even if it's supposedly in its best condition/mood, it could take you half an hour or more to get that damn fragment.
Finding all ten Wild Artefacts. You use these to unlock the optional time periods. Some of them are dropped right in your lap, but others require you to complete certain sidequests or are hidden well off the beaten path.
Finding the hidden monster crystals require you to throw Mog at specific points with a specific Fragment Skill active. Most of them are fairly mediocre, but some of them (especially Chichu) are excellent in their roles.
The paradox endings themselves require locking a gate and playing through a given section of the story again with the Paradox Scope fragment skill active. Some of them are fairly easy to figure out, but good luck with the rest.
Poison's secondary effect (nullifying Regen) is never mentioned in game. It's normally not that big a deal, but it makes several of the Paradox Ending boss fights against Caius much easier, especially the Oerba 200 AF rematch.
The hidden abilities for monsters. There's one for each role (Jeopardize for Commandos, Vigor for Ravagers, Reprieve for Sentinels, Boon for Synergists, Jinx for Saboteurs, and Curaja for Medics), and unlocking them is both a major Rare Candy dump and a total Violation of Common Sense. You have to infuse a monster from a different specific role (i.e. to get Jeopardize, you infuse a Ravager into a Commando), and the combined level of both monsters must exceed 99.
Secret Achievements/Trophies. You gain some of them by progressing through the story, but others require you to do specific things, which of course are hard to do if they're secrets.
Developing monsters that you catch and tame. The game offers limited information about monsters' upgrade paths, making development a matter of trial and (potentially expensive) error. The official guide has a different set of limited information, but does recommend some monsters for you. If you want to make your own decisions based on complete monster information, you'll have to try the internet.
The official guide obviously has an interest in promoting its own indispensability, but has a few passages of interest for this page:
Though the final boss battle tends to start promisingly, it can become incomprehensibly difficult when you reach its fourth stage without appropriate preparation and strategies.
Which the guide is happy to reveal.
The Strategy & Analysis chapter describes itself as having "a specific focus on details that the game itself does not reveal." It is not necessary for a first playthrough, but will repay close reading if you unlock Hard Mode.
Locating the Rare Merchant is something of a Luck-Based Mission with the help of the guide. Without the guide, you may not even know that he exists.
Secret achievements show up again.
Obligatory obscure sidequest: Completing the three-star quest "A rose by any other name" requires an item only available as part of the one-star quest "Voices from the grave." To start that, you have to speak to four specific NPCs during the day and then speak to three ghosts after midnight. One of the ghosts gives you the item.
Final Fantasy Crystal Chronicles is no exception, either. Finding a way to get through the barrier to the final level is only hinted at in a few of the numerous random encounters.
Let's just say the entire plot counts. Want to see the complete stories of De Nam, figure out what to do with the letter from Tida, or unlock the complete stories of the caravans? Want to get all the hints to finding the ??? chalice? Want to learn about the miasma and the crystals from the Carbuncles which are never actually mentioned in-game? Want to kill the Lich? Want to enjoy the incredibly rich and vast lore the game has to offer? Better shell out the cash for that guide, sucker!
If you chose blacksmith as your family's job background, you won't be able to get some of the best weapons or items in the game unless you actually create more players with different family jobs. While you don't have to switch players, you're clueless until you look online for help.
Also, Final Boss in his final form, the spheres of your family members, which are memories, are actually ultimate magicite displayed as ????, allowing you to cast Blizzaga, Thundaga, Firaga, Curaga, and Invincibility (they are randomized) without any charge time. However, to change the spheres into those spells before Raem eats them, you have to cast Cure on them and there's nothing in the game that hints you this. While it can be possible to beat him without those items, it can be harder and having most of your memories eaten results in a Game Over.
Let's not forget the very long side-quest that ends in you getting 1,000,000g for everyone currently in your party.
There's also a number of Artifacts that can only be obtained on certain maps, on certain cycles, and by getting a minimum number of points at the end of the level. The Life Ring, an artifact that replaces the same type of magicite, requires the player to be in Conall Curach and beat the boss with at least 302 points. The only reliable way of doing this is to play one-player with a Game Boy Advance linked to the second controller slot (it shows your bonus condition) and keep entering and leaving until you get "Kill Monsters" as your bonus qualifier. Then kill absolutely every single monster, because otherwise you won't have enough points to earn it. And then hope the RNG is in your favor, because it's only a 1 in 8 chance of spawning like any Artifact. Additionally, there game says that you can only get one of each ring, ever. The opportunity to get more for your other characters opens after 5 years in-game. Other rings also require specific conditions to appear, but nowhere near as insane.
Blue Magic in general can sometimes feel like this. Some of these spells are ludicrously powerful, reaching almost gamebreaker status, but these are invariably extremely rare, or even one of a kind. Didn't bring your Blue Mage along to fight the boss that has it? TOO BAD.
Final Fantasy VII has some of this with its relationship values. It was calculated by how many times you chose to speak to the other characters, and when forced to speak, what answers you gave. Avoiding as many conversations as possible with the female characters (including optional Yuffie) left you with Barret...for the one scene in the entire game that this affected. Completely pointless but for some dialog.
In Crisis Core: Final Fantasy VII, Minerva is MADE of this. This Bonus Boss requires you to look up guides every step of the way. Locating the best armor in the game will require this, in addition to learning how to craft the best fusion materia in the game that is mandatory to surviving more than 10 seconds in her presence. Did we forget to mention she spams One-Hit Kill Ultima spells that leave you barely alive even if you block it while wearing said best gear and materia in the game? You'll need a guide to BEATING her too. Also, her Limit Break Judgment Arrow disables Phoenix Downs. Thankfully, you can Mug her for 99 of them.