Okay, two part question: What's the dumbest thing you paid for on a vehicle and what's the dumbest thing you've done to a vehicle? I'll start:
-Many years ago, I brought our Corolla to the dealership for some maintenance. Toyota Sales rep noted we had 60,000 miles and suggested I get the fuel injection system cleaned. I asked the cost. $117. Sure. Got the car back and asked what, exactly, Toyota does for a fuel injection cleaning...was told they add some fuel additive into the gas tank. I had a feeling Toyota made $115 profit on that deal!
-Dumbest thing I've done to a car is, many years ago, was replacing a themostat in my Chevy Cavalier and got a little too macho when tightening the two bolts in the aluminum block and, yep, stripped one. Ended up having to rebore the hole bigger and had to replace the bolt with a larger one. (Turned a 10-minute job into about 4 hours.)
Dumbest thing I paid for: Extended warranty on my 87 Toyota pickup. That thing was a tank. Never had to do anything with it. I put 250k miles on it and finally sold it when my first child was born because the car seat got in the way of the stick shift. LOL!!
Dumbest thing I did to a vehicle: I built one of those homemade hydrogen generators that was supposed to make my Jeep have this astronomical increase in gas mileage. Long story short, didn't work. 
Ha! Yep, I foolishly purchased the Toyota warranty ($750) for my new 2005 Tacoma. Barely got to use it...at about 4 years and 10 months, the overhead temperature display died so used the warranty to replace it. Part was $315 and my deductible was $100. So I spent $750 to save $215....Toyota won that deal!! (Only reason I got it was because my friends said the Tacoma was the first year on a new Gen so it was smart to buy the warranty....they obviously didn't know of Toyota quality!)
put 5 gallons of diesel in my gas truck before realizing , stopped and topped it off with premium , it smoked a little but no issues
broke several wheel studs off my GF's plymouth before seeing the L on the end of the stud
it was nineteen eighty something....
In my younger years, I used to go to Jiffy Lube (may have been called QLube back then, I later learned how to do my own oil changes). I paid for an air filter and new wiper blades.....the most expensive filter and blades I ever paid for in my life. Nearly five times the cost of just buying those three things and installing myself.
Sometimes it's the little errors that annoy you to no end as long as you have the vehicle...................
Just the other day, wasn't paying attention when changing the cabin air filter on the wife's '13 Genesis Coupe. The slot cover has a tab at the right that you first insert and then push in the left side to clip. I accidentally had it flipped over in the dark to where inserting the tab was fine but when I pushed on the left side to clip it busted off the right side tab and the thin plastic slot it went into. Now I duct tape it in place.
My other doozy, was replacing the heater core hoses on a '03 F150 that I owned back in the day. I accidentally put my elbow and full body weight down where the cruise control cable snapped onto the throttle body and busted the plastic pin clean off. That was the end of cruise control in that truck.
Dumbest thing I’ve ever done was never checking the engine oil level on my vehicle and during a road trip in 2012 the Red ‘low engine oil pressure’ light turned on and I ignored it for several hundred more miles of driving (having no clue what that light meant), then on the way back home the engine seized on me and had to get a new engine (all my fault). That’s when I hunkered down and learned as much about cars as possible.
2nd dumbest thing was buying a used GM truck couple years ago - had to put $10,000 in repairs the first 2 years of ownership. I should have gotten a 4runner instead.
