I've done that before. Got gas and then bought a soda or something and totally forgot to pay for the gas and had to go back later to pay.
When I lived in Florida I went to a Publix, bought a bunch of groceries and forgot about a couple of 12 packs that I had stuck on the bottom rack of the shopping cart.
Got out to my car, started loading my groceries and noticed the 12 packs. Checked the receipt, saw they weren't there and literally ran back in the store to pay.
In Florida, they make the guys in jail work on the side of the road in those striped jail clothes. I wasn't about to be doing that!!