had a client who was at wits end with a problem
guy was losing his sh*t over a problem that "you *ssholes can't figure out!". To be fair it was a critical system, but in the beginning days of "the dot com era" there were lots of people with a little money, great ideas, and in spite of poor stress management and interpersonal relationship skills, decent people.
Resolved the problem (intermittent power loss) and spent awhile demonstrating WHY it was critical the UPS remain plugged into a certain outlet, made sure to demonstrate exactly what happened when it got moved to a lower rated one, and did so making sure the guy understood it. Let his rants run by and when he'd gotten that out of his system, he understood the problem, and apologized.
Also apologized with $300 in cash undeclared to my boss. And none of our future call outs to his company resulted in any bad attitude for at least as long as I remained with that outfit.