Pardon the lack of clear repro steps; I have already worked around the bug, and don't know how to retrigger the authentication process so I can get precise information, screen shots, etc.
I logged in and went to buy XBox points, and got hit with the authentication process. The process asks the user to receive a code at their email, and then enter it.
I didn't realize at first that it was using an old email address from when I'd first created the account, so had it send the code to the wrong address. I finally caught the issue, and used a link to get the option to resend to a new address. I got the code, entered it, and was redirected . . . back to the authentication process, but with the old email address. I went through the process again, and still was redirected to the authentication page.
I finally realized what was happening - that I was likely being redirected to the page I'd been on before selecting the correct email address instead of the page I'd been on before entering the authentication process - and just went to my normal account page to see if it was unlocked (it was). However, I wanted to inform you guys of the bug. I would assume that any bug that could hinder people from paying you money would be of interest :-)
Even if it's a known issue, you should know that people are, in fact, encountering this bug and it's impacting their buying experience. I almost gave up, since I was holding a young child and didn't need my Dance Central 2 songs imported *that* badly.