[slhck]

I recently learned about this when I reported Internet speed issues to my home ISP (upload was basically impossible, while download was at 100 MBit/s).

They said they'd look into it, but they couldn't process my claim unless they could prove something was connected via Ethernet to their router. (They apparently never trust customer WiFi speed test results, probably because WiFi on their crappy routers can be notoriously unreliable.)

I ultimately had to connect something to the router's Ethernet port, so I grabbed another WiFi router, configured it as an access point, plugged it in, and voilà, they could verify that a device was connected and processed my complaint.

Obviously customer service reps can easily get access to a list of what is connected to the router.

[book_mentioned]

One time the next-tier tech shut off my WiFi while I was troubleshooting with the entry-level phone support; I hadn't been warned this was an option or would happen so it really rubbed me the wrong way.