Not my first, but definitely my most amusing.
Right after I started last year, I knew I needed a new bike. I was riding a borrowed steel Raleigh MTB from the early '90s, and it was too small for me. A riding buddy and I stopped at the LBS during a ride, and i decided to try out a shiny Cannondale F500 (or maybe F600?). Anyway, it had toe clips - which I had never ridden but decided they would be cool to try. I took it out on the test ride and was having a blast - great bike - then decided it was time to take it back to the LBS before my brain to AmEx filter COMPLETELY dissolved.
So I'm turning around in the end of a driveway, and there's an old lady there watering her flowers. And 'm turning around SLOW because I don't yet have the "feel" of the c-dale. Well turns out to be a little TOO slow, and I realize I'm going to fall over. I go to put my right foot down and realized it's in the toe clips.
Much like Jumbo, I had a slow TIIIIMMMBEEERR, and I'm sure the old lady got a chuckle of watching my 430 pound butt collapse onto the concrete.
Not nearly as much of a chuckle as my riding partner got, seeing me ride back up with a bloody leg and arm but a thankfully very unscathed Cannondale. Both he and the LBS salesman got a very good laugh out of the tale, and the LBS salesman didn't even question me when I told him I'd rather not buy that specific bike.
Oh well, but it sure makes for a funny story