I don't know how this is possible since I thought PuzzleScript explicitly disallows this, but if multiple mouse-generated objects can be placed on the same tile, even if all of the mouse objects are on the same layer. Usually you wouldn't see this in practice since the convention is that you clean up mouse-generated up before the end of the turn, so usually only one is on the level at any time. Can be reproduced in e.g. the mouse input test game.

I probably want to continue to allow this for legacy reasions, but this should probably give a severe warning when mouse objects are not on a layer with only that object.