Quest this happened on: "He Ain't Heavy" in South Shroud
Description: When you go and save the NPC, a Goblin pops. You kill it and then speak back to the same NPC. If, while you are in the cutscene, another player triggers the goblin, there is a chance it will attack the person in the cutscene, thus kicking him off that cutscene and forcing him to try over again.