While playing Final Fantasy X the first time, I got to Old Zanarkand and inside the dome, I saved behind a sealed door in the Cloister of Trials. I wasn't savvy to good saving practices and after solving the puzzle, got into a battle I was completely hopeless to defeat. There was no way to walk back out the door from which I entered, the only way out was through; and with my existing inventory, spells, charged up aeons, etc. I absolutely could not defeat the boss.

It was pretty crushing because this moment is so close to the end of the game, but I gave up and had to come back months (years?) later to fully start the game over. I think maybe I played 99+ hours of Final Fantasy VII and then decided to give X another try. The second time, I was much smarter with maintaining multiple save games for safety. Not to be bested again, I grinded up all of my limit breaks and aeons for safety and completely obliterated the boss in one go the next time around.

I actually managed to soft lock a side quest in The Witcher 3 recently. If you loot a container right as a cutscene begins, the item will be removed from the container but not put into your inventory.

I managed to do this with a key (by mistake) and almost lost around 25 hours of gameplay lol.

Don't know if anyone has said it yet, but Fallout 3. There is a story quest where you have to ask a radio host named Three Dog information about your father and it's a percentage based skill check that if you fail it, I don't think you can progress (unless I am completely mistaken since it's been more than a half decade since I last played).

To make matters even worse, even at a maximum 100 in speech, the skill check can still be failed. Again, not 100% sure whether or not the Three Dog skill check is even required or if you can just run to the right place to progress the main story, but if you are a first time player you could absolutely screw yourself over not knowing about this.

In cave story, there is a decision around the middle of the game. If you make the wrong decision you can’t upgrade to the best weapon in the game.

I forget all the details, but i was annoyed when I found out about it.

I almost softlocked myself in The Evil Within (the first one). I've used up most of my ammo before walking into a boss fight and I just barely managed to beat him by using everything I had. It does give you ammo before the fight but it isn't enough to win, I imagine it would be easy to softlock there. I remember spending a huge time making sure all my shots land so I don't restart.

Takeshi’s challenge I think it was called. Notoriously bullshit game on the famicon

There are a LOT of these, especially from the 8/16/32 bit era.

https://tvtropes.org/pmwiki/pmwiki.php/Main/UnintentionallyUnwinnable

I'm sure others have mentioned it here, but..

Chrono Trigger.

I played Earthbound as a kid and got stuck in a golem without the item needed to get out. Since I got it used I didn't have the game guide. No idea how much stuff I missed but I never picked it up again.

XCOM: Terror from the Deep had some notorious research tree issues

I can't remember the exact method, and I may even be remembering the wrong game, but I think in Breath of Fire 1 there was an item that you needed that could be sold, or maybe not picked up, and if you didn't have it, you'd get locked out of a puzzle much later in the game. It was hard to fuck up, but if you did, it was 30 hours of game down the drain.

Ys 8 has a soft lock toward the end where if you didn't do enough side quests to build up enough affinity with your castaway group and party members you would get treated to a bad/neutral ending. Fortunately at that soft lock point there are enough ways to build up those points so you can progress past that point.

Little Big Adventure 2. Just before the last boss I managed to save myself on the last island without a way to leave it. But I needed to leave and get another Ball or w/e it was to unluck a door. It was my first real pc game experience ever. Dunno why I stuck with this hobby after that tbh :)

King Kong for the PS2 had a fire puzzle, where if you dropped the torch in the last section, you couldn't get a new source of fire. So you were stuck at a section where you had to burn away wood in the path forwards, but couldn't go backwards to get the fire.