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If two adjacent sectors have different heights, and an object in the higher sector moves onto the boundary (e.g. an imp killed with the rocket launcher, or a barrel pushed there by an explosion), it may begin to "blink" back and forth between the two altitudes.
When this bug occurs as a monster becomes stuck on a rising lift, the engine may decide that the monster is "mostly" at the lower altitude (and thus can use its ranged attacks), possibly resulting in the player's being shot at by completely invisible enemies.