I remember seeing the following interesting exchange recently on MathOverflow:
someone asked a question about something,
another person ($X$) answered (or commented) that the answer is such-or-such because of Foo's Famous Theorem about this-or-that,
another person ($Y$), working in a different field, remarked that the answer also follows from Bar's earlier result in a different domain and stated in a different language,
thereupon, $X$ realized that Bar's result was, in fact, equivalent to Foo's Theorem, and remarked something like “amazing! this shows that Foo's Theorem was already known to Bar decades earlier”.
I'd like to use this as an example of how different branches of mathematics can fail to know about potentially interesting results from other branches.
Unfortunately, I didn't keep a bookmark of the exchange, and I remember very little (and even what I did write might be somewhat altered by my inaccurate memory). I saw it fairly recently (maybe a few weeks ago?), but it's possible that the exchange took place earlier (I may have been browsing an old question), so it doesn't narrow it down. I also don't remember the topic (since I read a lot of questions about a lot of different subjects we can't infer much…).
Does anyone else remember seeing this?