He only did it because he snuck on the internet and saw where you wrote that he had never growled at anyone before.
Don't you know our dogs like to keep us humble?

Seriously though, don't worry too much about it. I mean it is serious, but you are looking for ways to nip it in the bud. That is not being a failure, trust me.

One other really good reward for when a dog lets go of a highly prized item such as a bone is to let them have it back again.

I would keep doing the trade game, and just keep it in the back of your mind that he may react again, so that you are ready for it.

Oh one other thing. Check him over to make sure he doesn't have an ear infection or perhaps a sore that you may have touched while petting him and suprised him so that he reacted that way.

Good luck, and never be afraid to post asking for help. It shows you care and that you're willing to work through things.
Or if you are afraid to post then at least PM someone you trust.