Having a bunged up spark plug at San Sebastian, where there are absolutely zero facilities. Of course it was one of the lower ones. It did eventually clear. An iridium plug, too. Since then I have carried a complete toolbox, enough to do a 50hr check, on every flight.
A dead starter motor when trying to start up for a flight to Brac, Croatia. It turned out to have been a circuit breaker which had almost certainly just been kicked when I was getting in (easily done).
Unable to start on my JAA IRT
This problem (on a cold start) is extremely rare. It took a fair bit of extra pump-priming to get it started. It has not done it again. I put it down to me having put in the cowling plugs after the previous flight, when the engine was still too warm, and the contents of the fuel system (after the fuel servo) boiled off. Normally I don't put them in unless the engine can be touched with fingers.