When I was into collecting WWII rifles, I grabbed a dirt cheap, beat up, never refurbished Mosin Nagant M91/30 from a kid in Chicago. I knew it was rough, and the price reflected it well, but what I didn't know was that on the left side of the rifle there was a HUGE gouge taken out of the stock. After going back and looking at all the pictures, I noticed the kid purposefully left that spot out. I called him out on it, and told him reputation is everything in the world of collecting. He made it right, but it was a shady thing to do.
Not a lemon, but I got a surprisingly good deal last year that made both of us happy, which is as rare as lemons. I traded a guy a PSA AR-15 I put together with a DPMS zombie lower and a small amount of cash for an older Arctic Cat 90cc quad for my kid. The quad looked rough, and I was nervous to do the deal. Luckily the guy knew his stuff when it came to working on things, and after I saw his workshop I knew it wasn't blowing smoke. To this day that quad starts on the first kick and rides like a dream. I ran into the guy a few months later and he said he loved the AR. Talk about a rare occurrence!