I've had mechanics tell me my cabin filter is bad on a vehicle that didn't have one.
I've had mechanics show me how gross my spark plugs were in a diesel engine sedan.
I've had mechanics tell me I need new tires on a truck I had just replaced the tires on two weeks beforehand, and driven in mud.
I know more about my truck's issues than the guy who just did an oil change and casually glanced at a couple other things.
All that said, I still appreciate an actual mechanic (not the lube guy who started on Tuesday) telling me what they found wrong. You ever know when you might actually miss something.
But for real, fuck that spark plug guy.