Once on the street on the very first bike I ever owned. It was a mountain road and I was a newbie. I was riding WAY over my head on a cruiser.
Three times on the track.
First time was definitely because of inexperience. I became overconfident and I happened to let my mind wander. This is VERY BAD for motorcycling, especially on a track day. I didn't look through the turn, I looked straight in front of me and thought, "****, I'm not going to make it," so I went off road. I then made another beginner mistake and target fixated on the tree row. I was trying to modulate the rear brake to get the bike to slow down but that wasn't working. As I got within 15 yards or less of the trees, I slammed on the rear brake, slid the bike sideways and laid it down on the right side. We both hit the trees and the bike stood up then fell on my leg. I wasn't injured, but I had a hell of a time getting the bike off my leg.
Second time on the track, I cut the turn too tight and the footpeg caught the track "curb" and the rear wheel lost traction and slid out from underneath me.
Third time on the track, I honestly don't know what caused the low side. I was taking the tightest turn on the track and was taking it slower than I'd taken it all day. However, the rear wheel just lost grip and slid out from under me. There could be any number of causes. I did find 3 slices in the rear tire in a later inspection, so it's quite possible that something was on the track, but I'm not convinced that was the problem.