Don't break into hjerim and take the Strange amulett and the beware the butcher pamphlets, before the quest Blood on the Ice starts!
I did, and so the quest never started and the characters involved started bugging out on me!Ā
I know probably not many are stupid enough to follow my footsteps but here's a warning, don't do it! And no, it can't be fixed with the "setstage" cheat.Ā