You've probably noticed the similarity between Madonna's "Express Yourself" and Lady Gaga's "Born This Way." Madonna has noticed it too. The two pop stars have seemed to be friendly when appearing together, but when asked about Gaga, Madonna once called her "reductive."
In this recent footage of Madonna rehearsing for her live show, she puts together a mash-up of "Express Yourself" and "Born This Way" that really highlights how similar the two songs are. In the context of what's already been said, it seems like a pretty clear shot at Lady Gaga. To make it even more clear, Madge finishes the performance off by singing a few lines from "She's Not Me."
Lady Gaga has kept her cool about this whole issue, so it's unlikely this will blow up into a rap-style beef, but we'll see if this is enough to warrant a response.