Did you ever check your case temperature? If it's not over-hot, the discoloration may have always been there or even been caused by a failure within the card itself.
I wouldn't be too quick to blame yourself, it's probably nothing to do with anything you've done.
Failures, although rare, do happen, and if your into gaming, you are placing heavy loads on the card, compared to say, a word processor user. I know these things are meant to be designed to take care of those loads, but as I said, failures do happen.