The server crash
So what happened was that the systems administrator from work emails me early in the morning asking me if I have any idea as to why our webmail isn't functioning correctly?
I take a look at it and offer the explanation that, because foo machine isn't getting data from bar server, a service on bar has probably crashed and needs to be restarted.
Time passes.
The systems administrator calls me up and tells me I was close, but not absolutely right. In fact, the servers... all of them... have crashed... to the floor!
I have on numerous occasions had to administer the machines in our "server room". Not once did I look at what the things were sitting on. Some sort of table/shelf Ikea half-breed. We probably would have continued stacking machines on it without a second thought.
As luck would have it the only thing wrong was that a cable had come loose in the fall. Plug it back in and everything works perfectly. Although were still going to hang on to that latest backup...