Shadowgate. Dear God, Shadowgate. Have fun with a first-person point-and-click adventure-type game with even more random instant deaths ("you take a look at the scroll, only to find out it's a scroll of explode and kill whoever tries to read it, nyah nyah") than NetHack, and sometimes the right answer is fairly obscure.
There is one room of the game with three mirrors. The only way to proceed is by smashing one of them with a sledgehammer to reveal a door that you open and go through. However, another one apparently has a magical black hole or something behind it as smashing it reveals a vortex that sucks you in to your death, and the third just has a wall, but trying to smash it somehow leads you to breaking it in such a way that the flying shards kill you. Hope you saved before having to take a complete unaided wild guess as to which one's which... And that's one of the easier puzzles.
One room has a pit with an obvious ladder affixed to one side. I hope you're not dumb enough to think that means you can climb down and that they won't just reveal afterwards that the ladder only goes down about an inch past what you can see from the top and then suddenly stops, leading you to fall to your death. Obviously the answer is to only come back much, much later, when you have a spell that can make ropes levitate.
Did we mention your character will sometimes gleefully commit suicide if you so much as look at certain items in your inventory the wrong way?
The Swedish translation of the game (a rare occurrence in those days) makes things worse by mistranslating one of the objects needed to kill the final boss. The only way to figure it out is by trial and error.
Actually made even harder by Nintendo Power. The room where you collect the staff has a rickety bridge where you fall into a bottomless pit when you try to cross. The solution in their walkthrough was so wrong that many who played it on the Game Boy Color for the first time were completely stumped by it. They recommend dropping all your items, which the game won't let you do in that room.
Uninvited was MUCH worse than Shadowgate. In the start of the game, there's a hallway where a mysterious woman appears that is actually a ghost and will kill you. The only way to beat her is to enter one specific upstairs room first and find something called "No Ghost" to use on her. Oh, and you have to OPEN the No Ghost before using it or it won't work; your character will just approach the ghost, shake a capped bottle at it, and get the hell killed out of himself. It just gets worse from there.
In a game like this, a huge ruby sounds like a totally awesome thing that you will need later, so of course you take it. Several minutes later, more than long enough to forget about it in the midst of avoiding all the OTHER death lurking about the place, you get a few cryptic messages about slowly having your consciousness encroached upon by a strange force. Then you die. The game never explains that the ruby in your inventory is the source of the possession, and if you choose to restart after dying of it, you will still have it.
This only applies to the NES version. In all other versions, this happens whether you have the ruby or not (although it takes much longer for the game to kill you in all other versions).
There's also the literal Ghost Butler upstairs. To defeat him, you need to use spray adhesive on an outdoor railing and trap a spider that walks by. Then, you use something called "Spider Cider" to knock out the tiny arachnid and take it with you. Then, you throw that little spider at the butler, scaring him and making him disappear. Some players may have figured out how to catch the spider without looking it up, but it's impossible to guess where to use it because there's no indication anywhere in the game that the butler is afraid of spiders. None.
At the very least, Shadowgate gave clues stating that the Staff of Ages, Golden Blade, Silver Orb, Bladed Sun Talisman and Platinum Horn were they keys to beating the Big Bad. No such clues were given as to how to finish Uninvited. So of course you would just figure out that you're supposed to turn on a bathtub to flood a bathroom and float up to the hatch above (disguised as a light fixture, natch), then hit your sister to release the demon inside her, THEN kill the demon with holy water from a goblet that you took from a church much earlier in the game!!!
And Déjà Vu is no better, simply put. Improper usage of any item in ICOM's adventure games is a game over.
In fact, it adds its own little twist. Some of the items you get are needed to prove your innocence. Others are the planted evidence against you. You'll need to figure out which is which, and discard the latter in the sewer. Miss one of the plants, and you'll get a nice shiny set of silver bracelets.
Star Wars: Bounty Hunter becomes this starting from chapter 3 (Oovo IV) onwards, pitting Jango Fett against hordes of well-armed opponents, snipers, Demonic Spiders and especially bottomless chasms difficult to navigate without a master control of the jetpack. Furthermore the checkpoints are very far from each other, you have limited lives (which can't be restored) and more than once you'll have to check any enemy you meet to make sure that he hasn't a bounty on his head.
Double Switch. Think of the game as Night Trap, but with the difficulty cranked Up to Eleven. For starters, this game has no timer, so you will have a hard time knowing just when an enemy shows up so you can trap him. You have to be careful not to let too many enemies escape, or it's Game Over. Some of them are top priority, because they will attempt to cut off your connection to the security system, and if they do that, it's Game Over. Some of them will try to kill the tenants of the apartment building, so you will have to trap them, or it's Game Over. As the game goes on, you will have to quickly switch between screens, and timing becomes vital. There is one point in the game where you will be given a trap, and you actually have to be at that screen to be told about it - while simultaneously saving a tenant's live in another screen. You need that trap, or it will be Game Over later on. There are points where have to keep traps disarmed and then quickly arm them again. Failure to do so results in Game Over. Yes, timing becomes increasingly important as the game goes on, and you will have to activate traps at some rather precise moments. Welcome to Nintendo Hard!
King's Quest V. The game is rife with dead ends, pixel hunts, rooms you cannot return to, and pixel hunts in a room you cannot return to to avoid a dead end later. If you didn't notice an item earlier, you'll get stuck and have no way of knowing what you're missing and where from. Very few puzzles in the game make any logical sense, and then there's Cedric the owl...
Wizards And Warriors. More specifically, the sequel. While you are not technically a One-Hit-Point Wonder, as you have a life meter, many enemies (if not the majority of them) can one-hit kill you, especially in the later levels. Your main (and often your only) weapon is a sword that has such ridiculously short range it's practically impossible to hit anything without getting hit yourself. Oh, and you only get two continues.
You didn't think a Visual Novel would end up here do you? However, the Dating SimSummer Session manages to do just that. It takes place in a high school/college setting and you have to get 1 of 5 girls and pass the final exam. This is easier said than done. The game requires you to balance your schedule to raise your stats and pass your exams. However, you need to shy away from this routine to meet the girls, you also need to work to earn money and such. However there is a stress bar that can make you fail your actions if high enough, and you need almost max intelligence to pass the final exam, and you only gain 2-4 intelligence per day if you choose to study, making it very hard to build up relationships with a girl and get an ending. Also, you can actually fail the final exam and get the bad ending even if you're smart enough if the stress meter was too high. Furthermore, building the right relationships with the girls can be a major Guide Dang It.