Exsessive memory usage.

[kilburn]

Limiting mysql's cache is not as bad as it sounds. Free ram will then be used as a disk cache by the OS, so if mysql disn't cache a query but the corresponding files are in the disk cache (stored in RAM), the query will run really fast. I tend to prefer the disk cache way because it can also be used for other things, while memory reserved for mysql can be used only by mysql itself...