I lost $400 in what seemed to me to be really stupid, although I still feel a bit cheated by it to this day. A few years back when I bought my house with an at-the-time-GF (which was a bad move as well) my dad decided to buy us a new battery powered lawn mower as a house warming gift. We picked one out at home depot - black and decker brand. Home Depot had a deal on at the time where you got a 10% discount on your first purchase if you signed up for the credit card, so we decided that I would get the card (since I shop at home depot sometimes) and put the lawnmower on it, and then my dad would pay it off with cash at the register right away. A few days later I discovered that the charger that came with the lawnmower was not working and had to take it back. They could not replace just the charger but would have to exchange the whole lawnmower package, except they didn't have any left in stock. Alternatively, I could return the mower for a full refund. I took them up on the latter option, got the refund, and bought the same mower at walmart for the same price. However, since it had been bought with the Home Depot credit card, the refund had to go back on that card which resulted in me having an extra $400 pre/overpayment on that card. Or so I thought...
A couple years later I realized that I still had that card with what should be $400 of prepaid credit on it, so I used it to buy a bunch of paint for a project (about $180 worth). I thought everything was good until a few months after that I got a notice that I had $180 overdue on the card (plus interest) and they would send collections after me. I called them up wondering what this was about since it should have been covered by the $400 that was available on there, and they told me they had no record of that transaction. I argued up and down, but they said I would have to go into the store with my receipt showing the refund. The receipt was long gone by that point (it had been several years, after all), and so it would seem that the money was gone. Poof. Apart from that, it also affected my credit rating.