I had a network outage here a while back.

I knew it wasn't on my end and after an hour or so I begrudgingly called them.

They assured me there was nothing wrong on their end and we spent an hour on the phone debugging my DSL modem, resetting everything in the process. When it still didn't work they suddenly discovered a mysterious and unexplained "network outage" that was completely invisible before they determined my modem wasn't broken.  So they fixed it.

But since my modem was "old", they sent me a new one. Just because. 

But I couldn't get the new one to work with my network here, even though I had every setting correct and identical to the old one.

So I threw the new one back in the box, and put the old one back online. It works fine.

As far as I can tell, it's because for some reason no one seems able to make home modems/routers/etc. that can interconnect properly and reliably with one another - even though the protocols are all fairly well defined. Different units from the same vendor don't work interchangeably, even if you set every single setting exactly the same.