In /var/log/mail.log, I see this after about 24 hours of postfix/courierauth.mysql being up

Aug 9 08:33:03 localhost postfix/cleanup[18492]: warning: connect to mysql server Can't create a new thread (errno 11). If you are not out of available memory, you can consult the manual for a possible OS-dependent bug

I upped my max mysql connections to 400, and 'processlist' only shows 6 mysql connections (when this is happening) Any idea what could cause this? (free shows that I have memory and swap free)

Just for kicks, I added this to limits.conf (just in case)
postfix hard memlock 500000
postfix hard fsize 500000
postfix hard nproc 400

Could anyone help?