This happened a few years ago when I worked retail selling cell phones and accessories.
We had a bundle you could purchase when you purchased your cell phone. It included a car charger, a cell phone case, and an extended year warranty.
A girl (late teens - early 20s or so) came in and purchased a phone. She bought and paid for a cell phone and a bundle. I forgot to put the bundle in with her phone. It was on the counter, ready to go, I just didn't remember to put it in her bag.
Our phone company, that does the cell activations was on strike at the time. It was taking anywhere from a few minutes to a few days to get a phone activated. I couldn't get the girl on the cell phone (obviously) and called her at home. She wasn't there.
I immediatly (sp?) put her bundle in the mail, after leaving a message with a family member that I would do so. I was off the next day.
Fast forward a week or 2. I come into work and am told about a customer receiving 2 bundles, and returning one. Turns out she never got my message and came into the store the next day. One of my co-workers gave her another bundle but never mentioned it to me.
This girl could have gotten away with it, but instead brought it back and was honest. It was customers like this that made my often hellish job worth it.