I'll start with one from a couple of days ago.
I get a call to go out to a gentlemans rental property to address a problem with his AC. He explains that he also gets no fan, and has replaced the thermostat, but that didn't help.
He happens to be an electrical engineer.
I stand quietly and do my best to appear interested as he lays out for me what exactly I should be testing and inspecting, and in what order, for a good 15 minutes. The breakers, or maybe it's one of the busses, or perhaps blah blah blah...etc.
Then I walked up to the furnace and plugged it in.
The look on his face? Priceless.