I was out riding with some girlfriends a few years ago and we had a car full of teenagers come up behind us and honk. My horse bucked, but I held on. The other horses were fine, thank goodness. I thought for sure my idiot horse was going to run out in the road and get us both killed, but it wasn't that bad.
Other than that, we've just had the usual jerks flying by at 55 miles an hour and not even giving us a wide berth. That's why I don't ride along the road anymore. People are d*ckwads.