Painkiller -the original (patched to the latest update)- has some annoying bugs, like (to mention a few):
- the auto-save system is not reliable, so make sure to always also save manually as often as possible.
- Quickload does not work on Windows x 64, the option even disappears from the controls screen!
- getting stuck between objects, in floors, in walls...
- enemies are immune to certain exploding weapons while another one kills them instantly
- you discover a secret area and jump to a secret; then you cannot return to the battle area because that is still sealed off untill the end of the level (Castle)
- from a distance, bullets don't come directly from the badguys but from where they were (for instance: the badguy runs to the left, his bullets are coming from the rightside) and while they're running, instead of standing and shooting (Docks)
- Objects (metal can, case) you move can just fall through the ground and disappear (sometimes with a water splash effect)
But a serious bug is in Military Base, the part around the rocket.
The nearest checkpoint is in the doorstep to the rocket. Take it.
Then: if you turn around to explore the environment and maybe find a secret, don't.
Because you will trigger 4 seperate sets of badguys around the two upper floors, and they will go after you going down. And that's what triggers the bug*. You'll end up with the upper door's fence closed while it should be open after you killed the last (2).
If the badguys die on the floors that they start (where they spawn), there's nothing wrong.
So: jump down along the rocket, floor by floor, to the bottom, take the secret and use the ladder to go up, and kill everybody. The badguys cannot climb ladders! So they will stay on their floor untill you kill them.
* It's entirely possible (and likely) that one last enemy gets stuck against / between / behind textures / objects on a place i can't reach, or even fallen through the map so he never dies.