When Joshua was about three he picked up a hat from a shop display while strapped in his pushchair, and Gina and I didn't notice until later. We weren't sure which shop it was from so we didn't take it back.

More recently, the scan-it-yourself till at Tesco's got confused and didn't charge me for all the items I'd put through. I didn't notice until after I'd got home. For the small amount involved (a pound or so) I'm afraid I was too lazy to make a special trip back to own up.