Stopped at the garage I always stopped at to buy my lunch on the way to work. For once, I also put petrol in my car there instead of at the supermarket.
Went in and bought my lunch as normal and totally forgot about the petrol. As I drove away, I remembered I needed sweets for a competition prize at work so went back.
Cashier laughed as I walked in and said, you realised then, I was so confused how she knew I had forgotten sweets. Wasn't until she mentioned £40 petrol I remembered.