Monster and block-removable obstacles
The toggle wall can also qualify under this definition, as touching a green button will remove the wall, but only temporarily until another button is hit. This is the only true "wall" which can be removed from the game; a removable blue wall is half acting dirt and half acting wall.
A movable obstacle
A block counts as a removable obstacle, since it can be moved from its square and is a monster-acting wall, and also is used in many actual play situations as an obstacle to other blocks sliding.
What they do in play
Most often, removable obstacles play a part in blocks sliding in chains, such as Madness II. When a sliding object runs into a removable acting wall, its reaction and the range of options afterwards depend on the medium of sliding. It cannot slide through a teleport, as the teleport would have been skipped; if an ice stretch is used, it will continue the other direction. The most important (often the only important) situation is when a force floor stretch leads to a removable acting wall.
If a monster was sliding, it will become trapped for good on this space unless the obstacle is a toggle wall, since any attempt to remove the obstacle will cause Chip to be killed. A block will therefore become unusable. When a block slides to a stop, it is still on the sliplist and will destroy Chip if he removes the obstacle, again unless a toggle wall was in front of the block. To allow the obstacle to be removed safely, a ram would be necessary. This will be officially learned in After the Rainstorm.