Medusa effect

The Medusa effect occurs when there is more than one wall patch in any "middle" wall texture of a two-sided sidedef that is visible in the display window. Due to a game engine limitation, animation in vanilla Doom will slow to a crawl and make play impossible until the offending wall is out of view.

The effect is named for the Greek mythological monster Medusa. Medusa was one of the three Gorgon sisters (the others were Stheno and Euryale). When they boasted that they were more beautiful than the gods, the gods turned them into hideous creatures with live snakes for hair. The Gorgons were so ugly that anyone who looked upon them turned to stone, hence the Medusa effect--"it's full of snakes and you turn to stone."