My “favorite” customer was a small business owner who we set up with an on-premise exchange server (by his request). He was adamant he didn’t want his e-mail hosted anywhere off-site.
He called in the next Monday, complaining about issues with his e-mails.
Turns out he switches off the power to his office building every Friday when he leaves for the weekend, to save money on electricity.
We got the exchange server running again and explained to him in detail what a server is and that it needs power to run.
He kept calling in every Monday, for months.
Eventually he started to complain about all the bills we sent him, saying he won’t pay anymore until we can finally fix his e-mail issue.
So we cut him loose. Half a year later, his business domain was up for sale.