The story below actually happened to me.
A few years ago I had a nice company car, and my wife had a rather tatty car we had bought second hand.
One lazy sunday morning she said she needed petrol for her car (only I was working so I always bought the petrol for both cars).
I got changed and went out the the cars, and thought to myself "I dont want to go in my wife's tatty car, I will go in my nice car".
So I got in my car and drive off to the garage.
Only when I was half way there did I realise I was buying petrol for my wife's car.