I've got a worst that turned out to be a best.

Around five years ago when the Music Recyclery was still in business in Illinois, I bought tons of games from them and one day they had a Turbo Duo with one controller for $139.99, it was a great price but they were shady and I didn't do my research. I sold a ton of valuable PSX RPGs to get it, I didn't get to test it with in 7 days and then told an employee that I bought it. He told me he wanted it but it was broken and they knew it and sold it anyway and didn't care. Sure enough, the controller was shot and the CD drive was broken. I sat on it forever trying to find a place to repair it, finally OSG repaired it and region modded it and I can sell it for more then my entire cost for it. It turned out good in the end and I found Super Air Zonk for $2 at Goodwill.

Bad

I knew a big time collector quite well and he'd let me take car loads of stuff home and make payments to him. I had just started collecting and I wanted Action 52 and Maxi 15 so bad. I paid $70 each from him knowing full well how bad they were and how much I overpaid. If I waited I would have gotten them at half that price.

Good

I went to Goodwill and saw several PSX longbox games for like 2 or 3 bucks each. I bought everyone on a binge and didn't know how much they were worth. I got home and found out that the X Com UFO Defense I bought was worth $60 and Ebayed it for just that!