Looking through the i25 patch notes, there's some more info on the coupon system.
Looks like it was something tied to the forums/website that the old SCORE server was working on, and you could use it to earn ingame rewards (such as the Bug Hunter badge). Maybe it was one of those things where you could earn "points" by posting that you would sometimes see on old forums? Obviously we don't have that enabled here, and I have no idea if that code is even still in place or not.
As for Bug Hunter itself... I always hated that badge, just the concept of it. Pretty much every other badge in the game has clear unlock conditions that can be repeated by as many people and on as many characters as needed. Even the hardest badges in the game are possible with the right method.
but Bug Hunter is different. You need to be lucky enough to find a game-breaking bug (while searching for one can help, there's no guarantee), it's ultimately up to the whims of the developers if you get it at all... and then even if you do get it, it's a one-time only method. That same thing you found in the game won't get anyone else the badge, they have to find some other game-breaking bug of their own! It's completely counter to how every other badge in the game works.
I recall on the old forums there were requests to do something like re-purpose the badge to make it easier to get. Make it something like (for example) an account badge that gets given to people who test things out on the issue betas. Something like that.