She’s pitching Cosmo magazine. The original source of this has already been removed from YouTube, then re-uploaded in a censored version, but some people had already downloaded it and preserved it for the ages. In the original YouTube version, the breast appears 31 seconds in.
Here’s a large, color-adjusted capture.
Here’s another version, different color treatment. (More realistic)
She will appear in the June Cosmo. Most, if not all, of the official shots were covered in today’s Daily Mail article, and/or this Imgur album. Cosmo’s official promo is seen below.
Guessing this “slip” isnt as much of a mistake as we think. They could have easily covered her nips for the shoot. (Not that I am complaining!)