Actually had a positive experience with GameStop just now.
Was doing a monthly receipt clean up and was looking at one for a bunch of trades I made on the 14th. Turns out they didn't scan my Mario&Rabbids for trade. I know it's easy to say that maybe I just misplaced it, but in this case I was 100% sure I had traded it in (because I had just picked it up from the Best Buy across the street *cough*) so I decided to call and see if there was anything they could do, expecting to get a ton of shit and a "too bad, should have brought it up before you left the store".
Guy on the phone figures it was a game they don't take in or something, but looks up my phone number, finds the game in only a minute or two and slaps the credit on a random pre-order he says I can just come in and cancel. I ask how much the credit's for and he cites me the regular pro value and I said I was thrilled to be getting anything, but ask if it was possible to throw in the $5 bump I'd have gotten and he does it no problem. What a bro, wish I could take a survey for him.
tl;dr If you ever catch a trade-in missing on your receipt, give them a call.