Poorly aimed irrigation sprinklers.
The guy in the black Ford Ranger who inevitably changes lanes at the last minute to get behind me at the red light at 5th and Oxnard Blvd. After waiting for all the traffic in the other lane to pass him while he is stuck behind me (he would usually be the 2nd or 3rd vehicle in that lane if he had just stayed there) he changes back into the other lane, passes me and yells out his window that I'm an idiot! I've lost count of how many times he's done this.