Tailgaters. I was terrified that if I slowed down too quickly they'd smash into me, so one time I took a turn too fast and crashed into a car I couldn't see because of a hedge, and the other I didn't brake quickly enough to stop for a guy who suddenly realized he wanted to take a left turn right then. I eventually told myself that if tailgaters crash into me, that's their problem for being so close behind me, and I just need to focus on what I'm doing.