Nothing so dramatic, but my first two remain with me:

1. The ubiquitous FS1E. Two day before my sixteenth birthday I'm *determined* to get my bike out. My mate (who is clearly even more stupid than me) decides to pillion it. We set off down a local farm track at, or close to VNE (35MPH). The front wheel hits the pothole from hell and bounces up. We slide backwards, resulting in the throttlette[1] getting yanked open and my mate sliding back onto the rear mudguard. Instant Tigger impression... Bounce, bounce, BOUNCE. Crash...

2. Same FS1E, now out on the road. Going along wondering why the steering feels odd. Lean into bend. Crash. Ahah! *That's* what a front wheel puncture feels like.

Just remembered another:

BSA Bantam. Again off-roading, happily trundling along a track. Front wheel drops into hidden dip. Bike stops. Rider doesn't. Did you know that the two prominent welded seams on a Bantam tank are *exactly* the same distance apart as the average pair on testicles?

[1] It was a FS1E remember?