It seems that in gaining over 100,000 players, Kevan’s Urban Dead game has managed to find a bug in MySQL that causes corruption of indexes. Unfortunately no-one at MySQL AB is able to replicate the bug so it can’t be tracked down and fixed.
More information can be found in the MySQL forum.

I’m now looking at installing PostgreSQL to see if that works better instead. It’s taking a while to compile the latest version though because I keep getting segmentation faults and internal compiler errors.
Although these are usually caused by faulty hardware, I’m not sure it is in this instance as the server already had the cpu/motherboard/memory replaced. I’m not entirely convinced that it was always MySQL at fault because I saw the occasional signal 11 message (segmentation fault) in the logfile just as it crashed.