Invulnerability colormap bug being demonstrated on E3M6: Mt. Erebus (see demo below)
When a player becomes invulnerable, the colors in their display change to inverse monochrome. In Vanilla Doom the sky retains its original color; long thought of as merely a property of the invulnerability power-up, this was found to be a bug after the release of the source code.
This effect can also be seen in Heretic (where an invulnerable player's display is golden monochrome), but not in Hexen (where invulnerability is shown by a rotating chess-rook icon), nor in games played in ZDoom (and presumably certain other source ports). PrBoom allows the player to enable and disable this bug.