Here's a little story that taught me to check my gear daily.
On my high-racer, I was coasting down a hill about 17 mph on a straight road and then came to tight turn. I slowed somewhat, and as soon as I entered the turn my wheels fell into a sharp dip built-in to the the road. The dip wasn't large or deep, but it contained jagged rocks which my tires hit hard. My rear tire went flat in an instant, causing me to lose some control. I zig-zagged a bit on the road as I slowly gained control.
I unclipped my left foot, but was unable to unclip my right foot! (I'm moving at very slow speed at this point.) I turned my ankle outward as far as possible--but it still wouldn't unclip. Thankfully, I was leaning just a bit towards my left, unclipped foot; otherwise, I would have went down hard to my right on to the pavement and roadside curb. Finally, I twisted my ankle outward with one quick, hard motion, and my shoe finally unclipped.
My right clip-in pedal looked fine. I then checked my shoe's cleat, and it was "spun" about 45-degrees--one of the screws holding the cleat was gone! I had no idea the screw had even been loose because I hadn't checked it in a long time. I've learned my lesson almost the hard way.
The coincidence is kind of amazing I think: At the exact moment I hit a rock causing my tire to go flat around a curve, the screw decides to fall out of my shoe's cleat.
David in FL