I had another cleat-related incident on the way home tonight. I thought I'd unclicked my left foot when I came to cross the road, but apparently hadn't, and I'd already rocked towards the left and was well and truly leaning that way before it became evident that my leg was still anchored to the pedal.
I fell pretty heavily on my left wrist, and it made a nasty crunching sound. Doesn't appear to be broken though, just sprained I guess.
On inspection of my left shoe when I made it home, it would seem that one of the bolts that holds the cleat to the shoe has come loose and disappeared. So when I turned my foot sideways, the cleat rotated on the other bolt, and didn't release properly, whereas I thought it had released by virtue of the fact that my foot moved sideways.
Just as well it's forecast to rain tomorrow. Don't feel much like riding at the moment.