If you trigger a rolling boulder trap for which a boulder didn’t generate because there was no room to put it, a boulder may fall out of the ceiling instead (and the trap is removed) similar to a cursed scroll of earth. On early levels, it doesn’t have to fall on the same square as the trap.

Also, rolling boulder traps (beyond early levels) that do find appropriate launch locations may generate with no preexisting boulder on one of those positions. When triggering the trap for the first time, a boulder drops out of the ceiling on one of the launch positions and then begins rolling across the trap.