Things We Forgot to Monitor
Also:
1) Maximum # of open file descriptors2) Whether your slave DB stopped replicating because of some error.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.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.5) If you're running out of space in a specific partition you usually store random stuff like /var/log.I've had my ass bitten by all of the above :)