I was riding along, clipped in, having a grand old time when my friend told me we'd missed a turn. So I tried to unclip and had a lot of trouble. My left foot came out fine, but my right foot just wouldn't release. Finally I got it out, stopped the bike, and turned around.
I got back on and tried and tried to clip my right foot in, but it wouldn't catch and...hmmm that's strange, why isn't it making the metal scrapey sounds?
SO I stopped, dismounted, and looked at the bottom of my foot. No cleat!
So I examine the pedal and there's my cleat, still firmly attached to the pedal instead of to my shoe.

I got out my tools and started trying to pry it off with one of my skinny wrenches. I wasn't able to get it, but luckily another biker stopped and pryed it off for me.

So I learned a VERY important lesson: always make sure your cleats are super tight on your shoes before every ride. Apparently they can come loose over time!