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, in vanilla Doom the computer will slow to a crawl and make play impossible until the offending wall is out of view.

The effect is named for a creature in Greek mythology. Medusa is the most well-known of the Gorgons. The Gorgons were created from beautiful maidens who claimed to be more beautiful than the gods. The gods punished them by turning them into hideous monsters with live snakes for hair. The Gorgons are so ugly that anyone who looks upon them will turn to stone. The hero Perseus killed Medusa by using a mirror to cause Medusa herself to turn to stone by looking at her reflection.

This bug was named the Medusa effect because of the striped pattern that appeared in the transparent portions of the texture and the reduced framerate. "It's full of snakes and you turn to stone."