Does not happen all the time but enough that it is REALLY annoying. I had two guys left to clear out a camp northwest of Jorvik and air assassinated the first pressed "f" and the moment the axe made impact it crashed. I had not done any looting yet as I always clear enemies first so i had to redo the entire camp to get the gear and ingot. This, I believe the third time this has caused a crash and the only thing that causes crashes. I am running everything ultra at 96fps so the game is running great save for the occasional bug (especially hovering in mid air flailing about while climbing - forces me to fast travel). I use multi-assassinations all the time so not being able to use it is very disappointing.
So far I am loving the game and enjoying exploring the new world. Only persistent problem I have had is using the horn to call for my longship. I have tried several various locations and distances from the water and ship with no luck. Hopefully this is fixed soon. It is not going to keep me from playing but it is annoying that such a bug exists upon release.