A bit of "live & learn" here, and is more along the lines of "What I did to my bike *yesterday*" but I digress:
I was trying to fire ol' Tempus up, peddling her up to speed, and right before I popped the clutch I went through a patch of very fine gravel (between "pea" and "sand"). I popped the clutch and, instead of firing up, the pedal chain popped right off and started dragging on the ground. Now, I knew it wasn't too slack before. Goodness knows I'm always tinkering and otherwise lavishing attention on my bike.
So I walk her over to the side and start trying to thread the pedal chain back on. And guess what I found lodged INSIDE an aperture of the pedal chain! A stone! Somehow, the front wheel had kicked a tiny stone straight back into the pedal chain and it must've landed just right and a sprocket tooth came along and tried to squeeze it in - couldn't - and it popped the chain right off! That must be, like, I dunno, a one-in-a-million thing maybe. Or maybe not; come to think of it, I have had times on pedal bikes growing up when I didn't know why the chain came off.
Anyway, I got the chain back on, and looked long and hard at its tension. All was good. So I climbed back on and started my bike and rode about with the local kids some, then I cruised down a nice, twisting, tree-lined street on a sunny afternoon, thoroughly enjoying the fact that I had nowhere to go and nothing better to do (already mowed the lawn, this was my reward to myself).