I had exactly the same on Tuesday, although a slightly different cause. I was on my old MTB in town, I noticed as I unlocked that I'd left it on the big ring so I pushed down hard when setting off and the rear wheel came out, left me on my back in the middle of the road!
It was a pretty quiet street so I got up and walked the bike to the pavement, someone walking past, bizarrely, said "I hate it when that happens", as if it was a daily event for him.
