The stupid freakin' sewers in Vampire The Masquerade Bloodlines.
You've just gone through a small house of horrors full of weird hoppy two legged mouth monsters with no humans to drain for blood. You've just fought a boss that summons more of them while teleporting around. And now...Sewers. FULL of those hoppy things and your only source of blood is rats (So if you're playing a Ventrue who can't feed on rats, you're screwed).
And then the sewers just keep going on and on and on and ON. So far on that there's legit 4 shortcuts back to the surface if you need to leave and resupply.
And then it has this super finicky water pipe timing puzzle where you have to make it through with almost second-perfect timing.
And then...Oh neat, a boss fight against this giant monster! Cool! ...Wait a minute...Oh no, now there's a maze segment with like 8 of them you either have to fight or sneak past.
And then you clear the maze path and...You need a keycard to leave. The keycard is in the maze segment, with the only clue being a one-time-only thing where a character appears and runs in a particular direction that leads to the keycard. So if you missed it, you're screwed and need to comb the entire freakin' maze crawling with baddies for it.
When I last played the game and found out a backdoor that lets you skip that whole giant segment and replaces it with a short run through a few tough monsters, I nearly jumped for joy.
Seriously, I can deal with boss fights that are punishingly hard if you're not built right on your first run, but those goddamn sewers are the part of that game that I DREAD every single time I re-install it.
Like, every other example I can remember of a bad part in a game is either just mildly annoying or just a letdown, or the game itself was just "alright" as opposed to great. Those sewers though...They really drag down an otherwise amazing game.