This is an obvious one but important... make sure you have log rotation set up. I had an admin who was in a hurry when doing an install and forgot to setup log rotation on a heavily used piece of software which eventually lead to the hard drive filling up. There should have been a low space warning email sent, the log rotation was should have been set up but because someone was hurrying, neither happened.

Not really a black swan, but I've found the majority of errors come from people rushing and not remembering to do everything because of workload.

take away: Checklists are a very good thing to have.