I used to work with internet service for trains. I remember one support ticket filed by the train conductor:

"Internet doesn't work"

No info regarding area
No info regarding when
No info regarding which carriage
...or even which train.

Well, I managed to jump through a few hoops, figuring out the schedule for this particular individual, and could deduce which train at least. Nagios reported everything as fine, but sometimes Nagios can lie, so I tracked it down and did the usual diagnostics onboard. Couldn't find anything wrong.

I phoned up the guy and it turned out he'd forgotten to enable wifi on his phone.