Riding: When your abiding by the laws of the road on your bike and some asshole/beehotch almost wipes you out in their car, then you pull up next to them and they flip you off/yell at you/ or snap you the dirty look when they fucked up in traffic and almost killed you!
Driving: Don't remember who posted it, but yes.. the asshat who speeds up from below the speed limit when you are looking to change lanes in front of them. I also hate how NOBODY is ever really paying attention to what they do in traffic. I know a lot of people that say they just drive around mindlessly and think its funny that they're terrible drivers. WTF?!