I've never been outright cheated, but I did encounter an honest mistake recently.
I had traded a PX4 Storm plus some cash for a beautifully engraved Rossi 92. The fellow who I traded with was the engraver. He had only bought the gun to engrave it. He told me from the get go that he had not fired it, but had cycled some rounds through just to check it. He told me everything the previous owner had told him.
So I got it, and was very happy with the trade, until about a week later when I broke it down to clean it. I removed the bolt, and noticed some epoxy on the receiver. I thought "Uh oh, that ain't good", and proceeded to remove this epoxy only to find a massive crack in the receiver. Whoever put it there had done a good job of hiding the crack, and it appeared as if the epoxy had been there for a long time. You really wouldn't have been able to notice it unless you completely broke the gun down, so I don't think the guy who I traded with had any clue.
I called him and told him, and he immediately wanted to make it right. He apologized up and down and sounded very thankful that I noticed it before I shot it. We met up the next day and reversed the trade. He was a very stand-up guy, who I think made an honest mistake. The guy he got it from pretty much told him he was stuck with it. I felt for him, but that was between them. I think he was able to get it fixed by Rossi for free, so I guess it wasn't too bad.