Combination
The combination tile, also known as the overlay buffer, is an acting wall that displays a mirror image of Chip or a monster on the last non-floor tile he stepped on. The tile is used internally by the game to correctly display transparency, and as such it was not intended for placement on the grid.
The tile has not yet been used in an official level pack, but it is used from time to time in some custom level sets, usually to confuse the player (as in Evan Dummit's level "Psychedelic").