A long, long time ago I was playing Doom (the original) late at night and I'd just finished a level, but decided to go back and grab a few more bullets and shells before I moved onto the next level.

So I'm running down a corridor, turn the corner, and an imp is standing right in front of me! It makes its trademark hiss and then slashes at me and I just about jump out of my chair.

I "knew" the level was empty, you see.

Right up until it wasn't.