The company I currently work for had a heat-related massive downtime issue a month or so before I started here. From what I understand, the AC failed, and alerts that should have gone out, didn't.

I caused some downtime once by making a mistake with the default login used by a linked server, but that's not the kind of downtime you're asking about here.

I think most of the downtime I've seen has been because of poorly set up and maintained hardware. But that was years ago with a sysop who really shouldn't have been in that business. Downtime was hours per month at that place.

The best "the whole network is down" I've been through was when a salesperson ended up with Slammer on a copy of SQL Express he had on his laptop, and plugged that into the LAN and brought the whole place to its knees. He wasn't even sure why he had SQL on the laptop, much less how he got Slammer in there. Took all morning for the admin to figure out what was going on.