I'll never forget it either, that one game that you thought was so good actually turned out to be a tragedy. It hurt so much that you nearly stopped playing video games altogether.
Or maybe it was because the game quality didn't measure up to the $59.99 price tag. The wallet sure as hell never forgave you for it, and to this day you're still trying to rekindle that friendship.
List the game or games that you regret buying.
For me it was Street Fighter X Tekken. I bought it on launch day too. Took it back after I discovered how awful the online was and the qualms of its DLC content.