System Shock has an annoying one with one of the revitalization chambers. The activation switches are typically located right next to the chamber, but for some reason Research's is in an obscure corner on the other side of the level's very, very large (non-linear) map behind a door that can only be unlocked by by destroying a significant number of security cameras. Most players simply assume that the switch just disappeared due to a glitch and never find it.
Near the end of Pathways Into Darkness, there's a teleporter maze, where all the rooms look exactly the same, square with a teleporter on each wall. There is nothing in the frickin' game that remotely hints at the path. Many other Guide Dang Its were also present, including the bomb code if you don't have the manual, the suffocation room; hint:use an item that speeds up time, the gauntlet of invincible green Oozes (a soldier who died from snakebite hints that they avoided him while devouring his teammates), the Violet Crystal(which is at the center of the randomly-generated Labyrinth), and opening the exit door, for which you needed to take the health-draining Artifact of Doom out of its box.
Even the official Halo 3 strategy guide won't tell you how to get the Skulls (at the behest of Bungie). While most of the Skulls are just inconvenient to track down, the "IWHBYD Skull" requires jumping through glowing rings in an order that plays the Halo theme, which is hinted at nowhere in the game, and then going back to the body of Truth.
It was actually first found out by cracking open the game code by some tech-savvy person.
The IWHBYD skull in Halo 2 is a guide-dang-it to get as well.
The above examples, and more, is probably why they made all Skulls available from the beginning in the Halo 4.
Because of Sturgeon's Law, Game Mods can sometimes suffer from this. One example is Eternal Doom MAP20: Silures, a puzzle level, which has a spectacular Guide Dang It moment near the end: To open the path leading to the exit, you must activate a specific tree like a switch, with no indication that this is even possible.
This is also true of the otherwise-excellent Alien Vendetta. On the first map, no less.
Memento MoriMAP 14 has a puzzle near the end where you need to press a switch to open a door, then run trough it before it closes. The problem—the switch is so far from the door that it's almost impossible to pull off this feat. Turns out, the switch is moved closer to the door if you step into a small, unassuming niche in a cave near the beginning of the map. There's absolutely nothing that indicates this.
The first two BioShock games had a hyperspace arsenal for weapons, but BioShock Infinite only let you carry two different guns. Burial at Sea went back to having multiple guns, but displayed them on a weapons wheel that was opened by holding 'F', while scrolling merely switched between the two you were using, so most players believed they had two guns when they actually had several. As you cannot pick up a weapon you already have, many players thought there was a bug where they could not pick up a weapon they had dropped.
In Doom 3, there are two special storage cabinets sent from a company called "Martian Buddy" that contain free stuff for personnel, and the codes to them are nowhere in the game. To find the code, you actually have to go to the website www.martianbuddy.comnote As of 2012, www.martianbuddy.com isn't even online any more outside Wayback Machine. Luckily the code is still available in FAQ's.. One of these allows you to obtain the chaingun early, big help for clearing out the Demonic Spiders at the end of Alpha Labs Sector 2 on higher difficulty levels.
Metroid Prime's Artifact of Spirit. The game tells you to seek the unseen entrance at the top of a certain room. The entrance you want is actually two or three platforms down in the room, hidden behind a completely normal square of wall. However, at the top is a Morph Ball tunnel that leads to a Power Bomb expansion. Combined with the misleading hint, this makes the hidden door way too hard to find. Just to add to that, the X-Ray Visor (which is what most people will think of when they hear 'unseen entrance') doesn't work well for seeing the hidden door.
The mining cannon in Metroid Prime 3: Corruption, which alternated between a mining laser and a vacuum beam while Space Pirates dropped in and tried to kill you. The cutscene did show the laser alternating between two modes (where the only difference that could be seen was between green and purple beams, big deal), but how was anyone supposed to know to wait for the beam to switch over to "purple mode" (the vacuum) before killing the last Space Pirate?
The Marathon series had some obscure secrets, but in the original, if you wanted to get the Flamethrower at an early level, you had to walk into a random corner of a maze to activate an invisible, soundless trigger to lower an elevator, sprint back to the starting point, fall down the shaft, grab the flamethrower and sprint back to the elevator before it reset. Failure to do so will trap you in the hole, with a terminal that says nothing more than 'And here you are, stuck in a hole. We could have done a lot together!'.
One of the most ridiculously difficult secrets of all time was the Deprivation Chamber, where you had to open an unmarked secret door, jump down a shaft,perform an amazingly complex Grenade Jumping maneuver to get up the shaft to a teleporter, and find your way through a completely-unhinted-at teleporter maze to find a secret terminal message.
Golden Eye 1997 falls under this in the Egyptian level. One of your objectives is to retrieve the Golden Gun. However, if you try approaching it directly, bullet proof glass seals it and indestructible gun turrets appear and tear you to shreds. The solution? You're supposed to walk across the floor in a certain path in order to get the gun without setting off the trap. The kicker? There is nothing in the game that even remotely hints at the solution! Even if you were to do the All Guns cheat and complete the other objective, you still need to go and collect the Golden Gun.
Getting access to many of the secret areas from Painkiller is a Guide Dang It moment. Requiring taking a Leap of Faith or exploiting the jump physics to reach otherwise unreachable areas, with the game not giving you any hints about where and when to do either to reach a secret area. Most times its better to leave well enough alone except to unlock the final difficulty requires earning collectible card powerups for meeting specific requirements when completing a level. Some levels require either finding all secret areas, or looting the contents of said secret areas to meet the card requirements.
Doom II has map 19, "The Citadel". In order to find a necessary key, you have to open a specific discolored wall in a generic corridor. Yep, you need to find a well-hidden secret area to finish the level.
This game isn't quite so bad as most though because unlike most later FPS games, it includes a map (possible thanks to the 2D level design), which shows doors, and can be upgraded with a powerup to show unexplored areas as well.
Star Wars: Jedi Knights II and III are almost always this—levels are huge and insanely complex, with inexperienced players inevitably doomed to suffer hours of running around in circles before finding out that a tiny button hidden behind a broken window or an inconspicuous console needs to be interacted with in order to continue.
There was one particular puzzle in Jedi Knight 2 where you had to power down a fatal force-field in a room to use a transit system, however the only way to find the power switch for the force field was to stand on a random balcony and look down, a pipe comes out of the wall at random intervals so if you look down at the wrong time you can't see it, once the pipe comes out you needed to jump on it, enter the wall and destroy the power source for the force-field.
In Jedi Academy, you get to put points at the beginning of every non-mandatory mission into whatever force powers you like. Among those is the force shield, which helps against some enemies, but again, it's entirely optional... until the mandatory mission on Vjun, where you land on a planet with acid rain draining your health in the open constantly. New players who didn't pick the shield will be confused at Kyle Katarn telling them to activate it, thinking it's one of the automatic force powers. The best part is, though, that you can't just restart the level and pick shield at the beginning - as mentioned, Vjun is mandatory, whereas only non-mandatory missions give you points. So your choices are replaying the entire previous mission, or suffer through the (rather extensive) Vjun level. Oh, the game also auto-saves after picking a force power, so you'll have to load a savegame from even earlier. Didn't save there? Time to replay two missions.
The first two Turok games had a strong tendency towards this sort of thing, since the keys required to access later levels were often hidden ridiculously well. The Chronosceptre fragment on the third level of the original is a case in point; after a very difficult jumping section, the player has to climb down a series of platforms that don't appear to go anywhere and make a jump to an area they can barely see.
Quake II has many secrets hidden behind nondescript walls that must be shot, with no hints whatsoever. A bane to those looking for the Last Lousy Point.
Deus Ex: Invisible War. The special weapons. Most are in out of the way areas you wouldn't otherwise think about, including a sewer, the bedroom of an apartment dweller, under some junk in an antique store, and a utility hallway. The worst though, is the Hellfire Boltcaster, which is hidden in a small room only accessible by jumping over to a small ledge in an area you don't have much inclination to be in anyway (it frigging off one faction to the point of sending assassins if you complete the objective there.)
Either a Guide Dang It or a fond memory, the loot in Thief: The Dark Project and its sequel could be fiendishly hard to find. For example, in the first mission of the second game, there are three coins hidden on a trompe l'oeil ledge in a stairway. You can't see the ledge moving up the stairway, and unless you have ninja eyes, you won't glimpse it coming down either, since the staircase is dark. The only way to get them is to move halfway up, stop midway, and turn around.
First mission of Thief II: The Metal Age has a particularly enraging secret (wedding rings). Even with a screenshot of it, this troper struggled to find it for twenty minutes.
Left 4 Dead and its sequel when it comes to playing as the zombies in VS mode. Newbies are shown the controls, but are rarely told how to actually use the special infected properly. Hunters can do a lot of damage to a survivor but experienced players know that Hunters are better in picking off a lone survivor or a survivor that is distracted while a newbie may try to pull a Leeroy Jenkins and pounce right away regardless on who is nearby. Boomers are made to cause surprise attacks by popping out of cover and puking, but newbies aren't exactly told this and may make the mistake of rushing at the survivors in the open, not realizing the Boomer can die in a single bullet.
Half-Life 2: Episode Two gives us the Hunter. It is very resilient to damage from most types of weapons, but has a crippling weakness to stuff launched by your gravity gun. The game never even hints at this, which is rather jarring considering how good Valve normally is at guiding the player towards these kind of discoveries; possibly, they assumed the player would just naturally try to fight it with the Gravity Gun for the sake of trying to fight it with the Gravity Gun. It doesn't help that the finale of the game requires you to kill them in droves during a Boss Rush.
Fortunately, they're also highly susceptible to Car Fu...
Revolution X. All of it.
First, the big one, finding the five hidden band member locations to "earn" the Aerosmith wings. There's one each on the Amazon, Middle East, and Pacific Rim stages. The Amazon one requires you to take an elevator (be sure to hit the button in time!), go to the third floor, which has five hostages, and blow up the grating above them before you're forced back to the elevator. The Middle East requires you to throw a disc at the nose of a sphynx...not the eyes, not the cheeks, not the mouth or forehead or beard, it has to be the nose...then pick up the disk that emerges from its open mouth. You have maybe four seconds to do this, and then you gotta do it two more times. (Did I mention you're in a raging battle against a heavily-armed bus at the time?) The Pacific Rim one requires you to blow up a specific crate and grab the item that emerges before it's scrolled off about two seconds later, and you have one crack at this.
So what about the other two? Well, guess what, they're on the very first stage! First you have to watch the direction the screen scrolls in the initial rooftop battle, then, once you're inside the club, head in that direction and enter the restroom (by shooting the sign); you'll find him in one of the stalls. And yes, half the time it's the women's restroom. The second is accessed via the lounge; you must traverse the row just behind the furthest front, then blow up the two fishes and the window below them in that order.
Also in the first stage, there's an armored truck that can do pretty heavy damage unless you destroy it quickly. Trouble is, your weapons can't pull that off. What can, however, is the hidden smart bomb accessible by disc-ing the boared-up window, which will take out everything except the missile launcher. Blow that and the truck is history.
The helicopter in the second stage causes problems for a lot of rookies, especially since there doesn't seem to be any way to destroy it. Here's the deal: Before the battle at the very end, you cannot damage it at all. The best you can do is shoot down its missiles; you're going to take a beating from its guns no matter what (thankfully there are plenty of power shakes in the level to offset this). When you face it at the end, then it will take damage...however, you must target the guns and missile launchers ''directly'. (Pounding the middle will damage bits of it but not destroy it.) Then you have to take out the mounts, either by targeting them or the bottom of the chopper, and then the bottom of the chopper, and finally the missile launcher on the very bottom. Anything else and you die a slow death.
In the Amazon stage, if you kill ALL the Everdrones in a wall, you get access to a hidden area with lots of discs and a few hostages. If you destroy the Everdrone-o-matic, this leads to another secret area. Trouble is, you need a lot of discs to accomplish either, so you'd better have saved up plenty beforehand.
About midway through is a scientist who hides behind a table and throws grenades at you. Only his arm is exposed (which you can't hit). The only way to take him out is to shoot the extremely inconspicuous chains holding up the extremely inconspicuous sign; the last piece will club him unconscious.
Oh, yeah, you know the final boss? That mutated insect thing that chases you all the way to the start of the level and fights you on the bridge? Remember how players kept pounding and pounding and pounding it and it just wouldn't die? How on earth do you kill it? Answer: You don't. Destroy the bridge supports on either the near or far side of the bridge, and this nuisance will be out of your hair for good.
The Middle East stage begins in a prison compound, with various hostages that don't give any telltale flash no matter where you shoot. What you have to do is hit the point where their sledgehammer hits the guitar at the exact moment it does so. Yeah, that probably should be worth more than 10,000 points, huh?
Finally, there's headmistress Helga herself. Shoot her, and she simply cartwheels away. Disc her, and she just flops on her back and insults you. Neither does any damage whatsoever. How, HOW do you kill her? Answer: You don't. Just disc her until she's as far back as she can go, shoot her to get her in front of the chair, and disc her one more time. This will knock her into the chair and reveal her...or rather his...true form, which can be killed. Hope you're not low on tokens at this point.
In Payday 2, the last day of the Big Oil mission is practically impossible without reading a FAQ first. You reach a basement full of similar machines, and you have to determine which one is a working fusion engine based on what's scribbled on some chalkboards, computer monitors, and very small notepads in the room. And if your texture settings are too low, they're unreadable. Trial-and-Error Gameplay usually doesn't work - there are dozens of possibilities, you move at a snail's pace while carrying them, you can only try one at a time, huge numbers of enemies come at you in an area with little cover if you didn't do a perfect stealth run, and the game won't let you finish the mission until you find the right one, even though other missions allow for incomplete successes.
It also doesn't help that the computer monitor that displays the information you need can be destroyed by gunfire or grenade explosions, which means you're shit out of luck if it happens and you didn't figure out the solution yet.
In E.Ψ.Ǝ.: Divine Cybermancy, the penultimate level when you side with your Mentor has you searching a Looter hideout for a wise and powerful Secrata member, Akmal. Dubious translations make the puzzle-based level extremely obnoxious - for example, a Looter tells you to shoot a "switch" to lower the water in a reservoir. Said "switch" is actually a metal cap on a pipe (in an extremely dark area) and must be shot twice. Akmal's incoherent ramblings almost make it worth the frustration.