Actually, the dumbest thing was also the best thing that ever happened to me (in reference to car maintenance)...
The last time I ever took my vehicle to an oil change place was about ten years ago. I made a specific point with the service "technician" to make sure to put SIX quarts of 5W30 in my vehicle, as required by the owner's manual.
I stood out in the area where the guy was working and after he put the new oil in (with one of those guns connected to the hose at the ceiling), I asked him (the same guy I just spoke with 15 minutes earlier) how much oil he put in there and what viscocity.
He said - and I'm not joking - "Five quarts of 10W30."
I reminded him of our earlier converstation, and he said something like, "That's what ALL cars get." He then argued with me about the capcity of my sump, ensuring me that it was only five quarts. He knew, of course, because he was an "expert in oil changes." I then removed the owner's manual and showed him in black and white that he was wrong.
He begrudgingly put in the one extra quart my vehicle specified. I was too stunned at his stupidity to force the issue any further.
And that, ladies and gentlemen, was the LAST time I ever let someone touch my car (aside from my trusted mechanic who usually does just the big stuff for me).