In the Catacombs, there is this room with the purple glowing mushrooms. From there is a stairway going down, leading to a broken path: you can jump all the way to the latest 'isle' and there will be an opening to another corridor.
That last jump is difficult; most of the times I failed there. And this brings me to the issue: when you miss a jump in the latter part of that broken path, you do not die, but instead fall all the way down to the bottom of the game world: an absolutely black floor (no bitmap assigned to it) and surroundings that match the Catacombs, so it gives the impression Tilo is in a big cave (btw the walls do not entirely match in the corners, having light come through the cracks, making it look a bit surreal — or glitchty, depending on your state of mind.)
Thing is: there's no way out of there! You actually have to load an earlier save., which imho is a bit pity, enjoyment-/experience-wise.
So, why not fix this? Either by making Tilo die when he falls (the least likeable, but most coherent option, because that is what happens when you fall at an earlier point), or create some sort of a shallow lake (accounting for Tilo not dying), with a long stairway up, maybe even with some non-vital things to be found over there. And/or maybe a leech in it, to add a penalty for falling. And/or some lore background things, whatever.