When battling the Act IV end boss as a "solo player" and another players enters then leaves the game mid-fight, you are able to re-summon your follower who was earlier imprisoned by the boss.

It may not seem that game-breaking, however it made it much easier for me to finish the encounter and I feel that it could be exploited on harder difficulties.
Edited by Blenderer#1584 on 5/21/2012 4:34 PM PDT