So up until yesterday I have never had a quest that could not be completed. I had joined a public game and a 4 man group to do bounties. I was working act 1 and had the Jar of Souls quest.
I killed all the monsters and destroyed the Jar and yet the quest did not register as completed.
Normally I play solo and have had this quest numerous times and had no issues. Perhaps the problem lay in the multiplayer environment