Well to return to the topic, here 2 stories from me:
Quite a few years back I finally convinced my mum to switch from dial-up to DSL after talking onto her about the advantages of speed and being able to call someone while being on the net and so on. Unfortunately she insisted on keeping the ISP ;-(
The transition went rather smooth, we got the new hardware and I choose to set it up my self as the technician way really expensive and connecting a few boxes isn't that hard after all.
I got to the point where the local network worked seamlessly and the router displayed the line as active and working but I could not get a connection on any of our PCs.
So I called the Support who was very friendly, kind and patient, but could not help me at all.
He kept insisting the problem had to be on our side and was using the phrase "the train is running on our side, you just have to hop on" constantly. He still wanted me to turn of the DHCP Server and use Static LAN IPs besides the web interface stated DHCP as the recommended option. (The fact that I could connect to the web interface, where he had me click almost every available option should have been enough clue that there was no LAN problem...)
I always got the same person which was nice as i called several times for several days, but could not find the problem.
At one point suddenly everything which had worked to that point (basically the phone itself and falling back to dial up) stopped working, too. This got me pretty worried as I slowly got convinced the problem may be actually on my side(and my mum nagging me how this situation could be an improvement after all.)
After being without a phone for 2 weaks it turned out that an excavator had capped the line in a really strange way which only cut of our house and none of our neighbors.
With that being fixed the phone and dialup worked again but I still could not connect to the DSL. Almost by trial and error on my side I finally got it running: I found out that our ISP did not even own the line but had rent it from another ISP where it had to be "unlocked" first by entering their login credentials instead of the ones given by our ISP once and then switched back to the other login data.
After confronting the tech with these findings he admitted that they didn't have their own lines at that point and that they rented every customer in our region at that point.
They could have told me earlier...
Now a more recent incident:
I had installed linux on my moms PC as it is user friendly enough these days (even for her) and basically disabled any settings and password protected everything what possibly could be changed by accident to prevent her from doing so.
This worked quite well for a surprisingly long time without error up to the point where she could no longer connect to the Internet. I was not reachable for a weak and so she started calling our ISP. I don't know what they told her exactly but she ensured me she told them that she was running linux more than once which didn't keep them from instructing her to press random buttons or try CTRL-ALT-DEL to "bypass that strange password protection which could be a virus of some sort". As the gnome menu reassembles the Start-Menu she could get to a command prompt by describing it entry for entry to the support guys (don't know how they figured what to click) where they made her enter strange commands which she unfortunately did not writhe down, I would really like to know which obscure "tricks" they tried to get rid of that "virus".
As I had a look at the problem it turned out our ISP had changed our logins which should have been translated to our router automatically (if I hadn't turned this feature off as one of my first setup steps) and all she needed to do was enter the new login credentials using the web interface. (Which she couldn't have done due to the routers own password) but the support also should not have taken some password prompts for a virus and trying to fix those kind of things remotely by entering random commands is very unlikely
I guess they just wanted her to stay in the line as it was quite expensive...