You know how sometimes it's a tribute to do a snippet of somebody else's song in concert? A form of hat-tipping, perhaps, or a winking acknowledgement, or simply basking in singing a song that you wish you wrote? Madonna splicing a snippet of Lady Gaga's "Born This Way" could theoretically be like that. But considering that the songsounds conspicuously like "Express Yourself," and which Madonna's carefully thrown shade at in the process of praising it... well, it's open whether this is a compliment or insult, and whether the "she's not me" bit is a gracious demurral or yet another jab. That's the mark of a true diva: always leaving things ambiguous, plausibly deniable, and letting your people interpret it as they will. Madonna knows.
That's one way to think about it. The other way to think about this is that the Internet is actively intruding upon real life, to the point where viral mashups just spontaneously emerge in people's concerts. This theory has a million holes in it. But it's fun to consider, right?
And here's a little explanation for those of you somehow still not quite sure what all that "Express Yourself" / "Born This Way" fuss was about. Listen, learn, be outraged on either artist's behalf.