I thought I should report this bug after I experienced it trying to complete the quest 'Take up arms'. I posted a thread concerning this already but have since worked out the specifics of the bug.
If you have a follower/companion with while working through the quest - in my case, Lydia - then while 'proving your strength' against Vilkas your companion will repeatedly attack him also. This results in a flickering of his red 'enemy dot' on the compass, him repeatedly drawing and sheathing his sword, his constantly yelling "Come on now, fight your own battles!" and your being unable to remove your companion's attention from him after the section is over - he/she will continue hacking at him. This leads to several other animation and AI bugs.
This is a pretty serious issue with the quest and can happen quite easily, so I hope it gets taken care of.
-Blackhole
