Twice.
The first time I was following the rider in front of me to closely, and I wasn't really able to see the trail,so I was following his lead. Well, he took a switchback way to fast(this was his first time on this trail, and also his first time in sand,which our trail has PLENTY of) and since I was paying more attention to how close my front tire was to his back tire, I didn't notice that he was slamming on his brakes until the last second, in which case I slammed on my brand new V-brakes and went up on my front tire for about 4 seconds, and then finished the endo. Fortunetly nobody was hurt and I had a few cool scrathces and lots of sand and dirt on my legs.
The second time we were on a road trip to a really cool trail called Lynches Woods. I took a turn to slowly and my tire hit a big root and stopped, but I didn't. Crashing, when unexpected, can be lots of fun!!