I'm not totally sure, because it was at the end of the second guild quest. This time was just farming for XP (I had already completed it earlier) but one thing I did do different was Megumin did her Explosion spell somewhere along the way, and there was a Dire wolf this time around. Either the Dire wolf or Megumin's Explosion caused it (or both) because at the conclusion of the quest it glitched out with that MASSIVE traceback and I had to reload. Please note that everything was fine until the end of the quest, THEN it glitched out.
I also had a traceback when Darkness tried to use a heal potion, but forgot to save that traceback. :( It wouldn't let Darkness do anything with the health potion, it basically just had her miss her turn.