I just hate how easy it is to miss things during the quest itself. I never found the amulet, so I never talked to Calixto, thus never getting the Necromancer Amulet for my mage who could have used it.
I was just wondering a day or two ago who would ever want to use such an amulet. Now I know
I remember getting some bit of dialog from someone in Windhelm on one quick trip to the enchanting table. I didn't follow up since I really dislike that city, and wouldn't have the house if it was free. Now there's a mod to put an enchanting table in the inn in Riverwood, I don't really need to go to Windhelm again. Ever.
>.> Isn't there...already one...inside the inn in Riverwood? <.<
Anyway, here are a few easy ways to break Blood on Ice prior to starting it (the few I can think of, not guaranteed breaks, but can potentially break):
-Breaking into Calixto's house
-Talking to Calixto
-Not listening to the conversation about the Dunmer that triggers upon first entering
-Completing the Civil War quest for either side
-Dark Brotherhood questine complete
-Death of Tova
-Death of Viola
-Having a bounty when the quest starts
-Using shouts in the vicinity of the quest to cause the guard to scold you for shouting
-Talking to anyone related to the quest prior to the quest starting.
-Frequently fast travelling through the area and not checking the graveyard
-Reporting that Dunmer salesman to Viola for stealing her ring
Those are just a few reasons I've run into with this quest breaking from reading the forums and personal experience, but I'm sure there are more. Basically, just chill out in Windhelm on your first visit and make sure the quest is the first damn thing you do when there.