A bug I noticed today:
Occasionally, when a car's skin has not yet loaded in multiplayer, the model of the car will be messed up (polys going every which way). As soon as the skin loads, the model snaps back into its correct shape. I've tried to capture this in a replay, but it doesn't seem to happen in the replays (possibly because the skin has been saved by the time you view the replay?).
I know this is vague, sorry.