The Medusa effect occurs when there is more than one patch occupying the same column in any middle texture of a two-sided linedef that is visible in the display window. Due to a game engine limitation, these will not display properly. The area where the middle texture would be displayed is instead a series of multicolored horizontal lines. This 'wall' of multicolored lines appears to extend infinitely into the floor. Moving close to the wall will make the framerate in vanilla Doom slow to a crawl and make play nearly impossible until the offending wall is out of view.
The effect is named for Medusa, the creature in Greek mythology who had live snakes for hair and was so ugly that a single glance at her would turn the beholder to stone.
This article is based on information in the Unofficial Doom Specs.