Disclaimer, I did not try this on other difficulty, so I don't know if it's an exclusive NM bug or universal bug. Also, since I don't like dying, I haven't ran this senario 50 times. All I know is that my sorc never killed NM Duriel. That alone suggested a bug that might not be obvious or repeatable (at the very least)
Okay, so here is what happened.
I killed Summoner while he killed me. However, I received a message all of a sudden to talk to Jeryn, and he told me to have the Ship Guy take me East.
I think several conditions:
1) I did not engage any character in dialogue as I completed quests. (Thus, maybe the order of conversation, the loop, was somehow messed up)
2) I did not check the quest log even though I completed quests. (I don't know if this makes a difference, but if it does, then same reason as above)
3) Die the same time Summoner killed me. (Really hard to do if you melee, senario can be repeated much easier with Sorc or Zon... I would imagine)
I think those are the 3 requirements. This was not a connection/lag bug thing since I can't imagine how not killing an Act Boss has anything to do with lag. I never had this bug before (then again, before was 3 years ago), and I don't know if anyone else got this bug before. Figure I share it.
This I also found. While killing in Hell (I think I was in a Baal Run), I died due to lag. However, instead of respawning in Town, I respawn right on top of my body.
This is a minor bug and I THINK I know why this happens. I think it had to do with the time lapse difference between the game data stored in game server and the actual real time game play. Basically, the two data did not synchronize due to lag. I think Blizzard might have forseen this problem. Thus, instead of crashing the game, they programmed it to skip the process, catch the error, and fix it with temporary "fix" codes. (You know what I am talking about, COMP SCI majors) And the end result is that.
(also, putting 2 to 2 together, I think this might be the basic concept behind duping before Blizzard made a server side patch to drop any games with super high latency. Thus, preventing the exploitation of the failed synchronization bug)