Problem
Cursed potions of -stat- (strength, learning, beauty, etc.) can reduce its potential from 1 to 0, and into the negatives if the PC is cursed and/or doomed.

How to reproduce it
With a stat and its potential both at 1 point and a PC without any bad luck intrinsics, drink the corresponding cursed potion of -stat-. It will remain at 1, but its potential dips below that, to 0. Now put on a ring of doom and drink another cursed potion - the potential becomes -3.

Notes
This is extremely unlikely to happen during regular gameplay.