Savegame buffer overflow
Vanilla Doom defines a hard limit on the size of savefiles. On some large levels (including MAP24 of Hell Revealed and a few others in Requiem), due to the number of Things, this can result in a crash when the player tries to save the game. The limit seems to be about 180KB; it was raised in v1.4.
Because the number of Things in a level changes during play, it is not generally possible to identify this bug without testing. Certain editors, such as DeePsea, can estimate the size of a savefile for each difficulty class.