I was working with customer today which has MySQL on a system with some 64GB or RAM running MyISAM, so they set key_buffer_size to 16G… and every few days MySQL crashes.
Why ? Because key_buffer_size over 4GB in size is not really supported (checked with latest and greatest MySQL Enterprise 5.0.34). It works just fine until you have less than 4GB worth of key cache used and then it would crash.
If you do not know about this limitation it may be quite complicated to figure out what the problem is as MySQL will be crashing on random statements associated to random tables.
What I find interesting is this bug was known for years and not only fix was not made available but even maximum key_buffer_size was not restricted to safe limit. Former would be close to changing single line of code with variable values limits, might be a bit more than that but it should not be that bad.