I have one user whose posts are crashing mySQL. This is happening on a shared hosting environment, and MySQL stops working for every other site as well. I have also noticed that when people read some posts or comment on some of her posts, MySQL crashes too! It doesn't exactly crash. The processes are running, but nobody can use it. I have to kill MySQL and start it back up again.
When I look in the mysql logs, it's always her editing or posting just before the trouble begins!
We're on version WPMU 2.7.