Apart from corrupted data files and variations on that I don't remember being able to (repeatedly) having been able to crash MySQL for years. But today I could. Darn, I was looking forward to changing my production MyISAM tables using spatial keys to InnoDB. Not today I guess. :-)
update So that bug hasn't been fixed yet, but the busy bees at MySQL did fix another bug I submitted a while ago. Whee.
Hopefully the bug will be squashed soon ;) It's good that MySQL managed to duplicate that bug :)