A looping respawn of monsters for about 3 minutes, roughly 60 monsters of the same type (not illusionists).
On Desolate Sands in Act II Hell during the Blood and Sand quest just north of the waypoint where the path widens into the open field. This was during a 2 player game. I could not duplicate the bug.
While searching for the 'Blood of Zultun Kulle' I came across Stinging Swarm (I believe that was the name). At first there were a pack of 3, and I quickly killed them. When I killed the 3rd of the pack, another pack spawned. This process repeated until we had approximately 60 kills (more than 46, as that was my friend's kill streak, who joined the battle late; I was tabbed out at the time and didn't see my kill streak), and then the "bug" ended.