Several bugs are present with no monsters mode. On some levels, monsters are a necessity for exiting the level, such as the Commander Keens on MAP32: Grosse (Doom II). Also, the Icon of Sin's monster spawner is ignored by the -nomonsters command line argument. (When such a level is completed, each kill aside from Romero's head adds 100% to the kill percentage.) These bugs are even present in modern-day source ports.
- All maps with Tags 666 or 667, such as Doom II's 32nd map or E4M6: Against Thee Wickedly (Doom), cannot be finished with the -nomonsters mode unless the command line argument -timer is used or the player uses the no-clipping cheat code. In that level, the four hanged keens at the end of the map will count as a monster, thus being included in being removed by the code. Since killing the four Keens will activate the switch, it's impossible to finish it unless the player turns on no clipping mode and triggers the switch by accident, or if the previous procedure described in the first sentence is used. All boss maps in Doom, such as E1M8, cannot be completed as the bosses won't appear in the map. E1M8 and E4M8 can be completed by using the no clipping mode, while completing E2M8 and E3M8 require the usage of that command line argument.