Just after I passed my test I had to put petrol in my (mum's) car for the first time.
I was pretty sure I knew how it worked, having watched people do it many times before. So I park next to the pump, get out, stick the key in the lock, turn the key and try to remove the cap.
No deal. For some reason the cap would just revolve, without ever unscrewing. So, I put the key back in, lock it and unlock it again and keep trying, but still no success.
I kept fiddling for five minutes, trying to lock and unlock it multiple times, while my bother, who was sat in the car with his girlfriend of the time got more and more embarrassed. I even tried asking a bloke who'd just driven up but he had no idea.
It wasn't until I tried locking the cap and then turning it that I realised what was happening. My mum hadn't bothered to lock the petrol cap, so every time I'd "unlocked" it, I'd actually been locking it, which made it freewheel rather than unscrew.
I did feel like a bit of a numpty.