I stole a whole tank of petrol once. Filled up, assumed my friend had paid (no idea why; they'd not gone into the shop!) and drove off. Realised later when I asked her how much it was and she looked at me like I was a nutter! 
Went back the next day. They had my registration details in a little book with the amount and a timestamp. I paid, they crossed it out, no issue thankfully!