This is where I first read about it and it seems some people got confused or forgot how Gearbox explained how the keys worked. Most thought it was probably 1-time use per character but its literally just 1 key so if you use it for one character class and want to play with a new character or save file, the key doesn't reappear.
http://www.screwattack.com/news/noti...-borderlands-2