Sounds like an experience we had once on a group ride. A guy latched onto our ride who was not part of our regular group. He had zero riding skills and kept slowing down, speeding up, racing to the front of the line, riding squirrely, etc. Finally we told him not to ride with us because we were afraid he was going to cause a crash. That ticked him, and he sped off ahead of us. Unfortunately, he was a lousy climber so we would catch him on every hill. Towards the end of our ride was a moderately long and steep climb, and he dropped off the back quite a ways. The next 2 miles of the route was relatively flat, and after a while I noticed him trying to catch us again. There are some bad pot holes on this stretch of road, and we pointed out the holes, as we always do. This guy was so intent on trying to pass us that he did not notice the holes and did not pay attention to our warning (or didn't hear it). He was still about 50' behind us when we heard him yell "Oh S**t!," and then a crash. He went down hard on his side, right on the pavement. We rode back to help him and he was all ticked off at us, like we caused his crash in some way. He obviously was hurt pretty badly, but kept trying to stand. I held him down and told him to rest while we called 911. He didn't want us to call an ambulance because of the cost, but we called his wife on a cell phone, and she gave us the go ahead. Good thing. Turns out he broke his hip. The only fortunate thing about this incident is that he didn't take down a whole paceline. My group has been very cautious about letting stray riders latch onto our rides ever since. If someone is at all sketchy in riding skills, we tell them to ride apart from us.