I don't know if it's the recent patch, but this has happened to me 3 times so far in the eastern part of skyrim.
I'll be randomly running through the wilds, see a dragon in the distance, decide to try to catch up to it to smack it around and get some gold. It'll be a blood dragon which will start flying backwards and cause my game to lag ~4FPS
Another note, I have this game on the Xbox360 - not the computer. Wish it'd let me turn the graphics down or something. One time I managed to "kill" the dragon, but instead of falling to the ground it kept flying backwards in a circle...
Anyone else have an issue like this? Makes me wish I got skyrim on the PC. Perhaps someone would've had a fix for this by now.