Summary: Illusionist copies remain on screen after the champion/rare monster is dead.
This has happened two times, both in single player. If an illusionist monster is killed while the copy is off screen, it will not despawn until you leave the area. It does not attack or move, just remains on screen. This was on the mac version of the game.
Link to a screen shot of the glitch: http://i.imgur.com/M6UWO.png
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