Luckily I have never caused an accident that involved another car. However, I've had a couple of people run into me while I was waiting in traffic or to turn.
By far, the worst situation I caused was this: I was driving to Chicago, going to get a visa for a trip to China. I was accompanied by ecib, actually. I was passing a semi-trailer in the right lane, and hydroplaned on a puddle of slush. ecib was in the passenger seat, and we slid under the trailer, him first. I was pretty sure we were going to be gravely injured or dead.
Luckily, we spun a bit, and were kicked out from under the trailer, maybe because we hit the jack stands, or the hanging spare tire underneath. We then spun around some more, slammed into the median, and came to a rest. ecib was covered in shattered glass, but we were fine. The semi never stopped. I'm not sure he even knew what had happened.
We called the cops, got the car towed, walked in the snow to the first L station into Chicago (which was just about 2 miles away), got the visa, and took a Greyhound bus home.