Old versus New
We had some performance issues recently and discovered we were using memcache-client 1.5.0. Our daemon was blocked by memcached socket operations hanging forever, which caused monit to kill and restart the daemon after 5 minutes.
I performed the brain surgery to use 1.7.4 (since we are using Rails 2.1) and deployed. See if you can guess when. Graph shows operations per hour (more is better).