Armor percentage rollover

In versions of Doom prior to v1.2, armor percentage was not limited to 200%. Since the ARMOR count in the status bar has only three digits, the display can wrap around to 0% if enough armor bonuses are picked up.

In theory, if the player continues to pick up armor bonuses, his actual armor percentage will eventually exceed MAXINT (7FFFFFFF hex == 2,147,483,646 in vanilla Doom) and wrap around to a negative value. According to the algorithm which transfers damage from health to armor, this would cause the next successful attack to kill the player. In light of the many other overflow conditions possible in the Doom engine, however, a level containing so many Things is almost certainly infeasible (even assuming a computer with sufficient memory and disk space).

Source

 * PrBoom listserv post by Andrey "entryway" Budko