Attacking citizens. It's seriously addictive. I'll shoot 1 just for fun, end up with around 8k bounty, quickload and start over. Other bad habits include, but are not limited to: loot hoarding, pickpocketing to the point of over-encumbrance, trying to converse with fellow vampires, death by fall damage, and wasting all my arrows shooting the solitude hawks (which seem to have developed a force field that the arrows stick in).