Today (day after patch 5.1) I decided to start leveling my lvl 85 Hunter to lvl 90. So I did some quests and decided to do a quick dungeon. I queued up and when I entered I found myself earning alot of achievements, which I had already earned on my main. I didnt think much about it until I saw that I had new mail. I got 2 new mails, one with the Black War Bear which I couldnt learn because I already had it, and another with the Vicious War Wolf from the "Veteran of the Horde" achievement.
Now that is where it becomes a problem, it just went from a random bug to a potentially exploitative bug. As you might know you get only one Vicious War Wolf for just one character per one of the 3 "Veteran of the Horde" achievements (75-, 150-, and 300 RBG wins). But because of this bug, if you have already earned Veteran of the Horde achievement you can potentially get the Vicious War Wolf on all your characters which is an exploit.
Now this means that it can and most likely will be exploited to get free mounts for all your alts without having to do any more of the achievmeents! The mount is not meant to be account-wide. If Blizz wanted it to be account-wide they would have made it account-wide to begin with.
Posting this here in hopes of more people knowing about it and reporting it so it gets fixed.
Edit with video of it: http://www.youtube.com/watch?v=XkO3xneq354