Overlapping units

[temple]

Usually units collide with each other, with the exception being units in the same formation. If you stop and disband the formation while units are in the middle of rearranging, some of them may be overlapping. When you now move them, they'll test for collisions against each other but TestRayAASquare doesn't count inside-to-inside collisions, so they're free to move through and on top of each other.

This situation can also happen when units build something and aren't close enough for the foundation to trigger a leave foundation command, but are close enough so that moving onto to the building is inside to inside and isn't counted as a collision.