I work in IT. More specifically I work in Networking. So every fucking day.
Best one was when I was working for an internet provider. Customer was complaining every other day their internet was not working to spec. They ordered a 100Mb service and could never get more than around 60Mb. Somehow they got it into their head that our on prem gear was the issue. But our on prem gear was gigabit capable.
But they never listened and eventually it got to the point where ceo to ceo calls were happening.
My ceo managed to make them agree to a call out where if no problem was found in our gear they would pay the $120 callout plus time for the 2 hour drive to the customer office but if I found a problem on their side we would compensate them for the services until it performs as ordered.
I get there plug my laptop into their network and sure enough I get 60Mb speed tests. Moved my cable from their firewall to the spare port on our gear and get the full speed.
They wanted me to do the tests a few more times and I do and get the same result.
I looked up the spec sheet of their firewall and third line down of the throughput graph shows their setup only has a rated capacity of about 60Mb. Never heard a peep from them after that.