Things We Forgot to Monitor
Also:
1) Maximum # of open file descriptors<p>2) Whether your slave DB stopped replicating because of some error.<p>3) Whether something is screwed up in your SOLR/ElasticSearch instance so it doesn't respond to search queries, but respond to simple heartbeat pings.<p>4) If your Redis db stopped saving to disk because of lack of space, or not enough memory, or you forgot to set overcommit memory.<p>5) If you're running out of space in a specific partition you usually store random stuff like /var/log.<p>I've had my ass bitten by all of the above :)