When an NPC in an old (S&D) assignment map dies after you already turned the quest in to the NPC, it will trigger the progress from the past quest, making the NPC say "I believe you have accepted an assignment elsewhere already, Finish that one first." and the only option is "Ok."
Won't be as big of a problem after the assignments begin despawning again, but it is still bound to happen. Perhaps add the ability to abandon that other quest that is detected in this dialogue tree?