Playing AssCreed2, I was hunting one of the targets of the second bunch, the guy who was shouting things from a huge tower (could anybody even hear him down below?), and I was just about to finish the guards off who were patrolling right under him. I had just picked up and thrown a body off the tower, and was about to go assassinate another one, when my target jumped right down in front of me.
Scared the living hell out of me and made me automatically try to assassinate him. Only problem was, I just picked up a body, which reset my weapons to my fists.
I ended up punching him twice and sending him flying off the tower. Made me stand there for a few seconds wondering "was that supposed to happen?"
Went into a cutscene of me stabbing him anyway.
Same game, but not a boss. A guard whacked me off a ledge in Venice, then jumped after me. I grabbed a ledge as I fell and chuckled as he landed right in the middle of a canal.