Dell sent me an extra monitor back when I lived in the dorms. I called them up and they demanded I send it back or they would bill me. It was really frustrating because I had to walk 1/2 a mile carrying this damn monitor in 100F humid heat and then pay $20 at the UPS store. They wouldn't even send me a label. I got screwed for calling them up and trying to to the right thing.
A related story. I accidentally left a little package of blank CDs in my cart while self-checking out at Wal-Mart. I walked back into the store and the guy tried to give me a little returned merchandise sticker. I told him I accidentally didn't pay for it and came back to pay for it. Security got called, I got brought into some holding room, they threatened to call the cops, etc. They treated me like a thief, but I was just trying to do the right thing.
Blegh.