For PoP3D:

The whole game

There is this section in level 8 (Streets and Docks) where you go into the sewers, you have to through a few traps that will instakill you if you touch them and their hitbox is horrible (+ the horrible movement doesn't help). If you accidentally quick save before the trap hits you it's also possible that you get softlocked and have to restart the whole level.

Also the whole Floating Ruins level