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 (who had live snakes for hair). A glance at the Medusa turned the beholder into stone.