The "Clock Bug" is a bug in Hexen.
This bug can be observed by this way: first, go to the clock room in Castle of Grief and fix the clock with 4 gears. Then look at the clock and wait for a while. When the clock shows "08:45" (for instance), save your game. Then load your game again. The clock will show 00:00, despite of the time it displayed before saving your game.