In Part 2 of Elseworlds, when Felicity finds out that Barry and Oliver have switched bodies, she says "it's not every day you find out your husband is your ex and vice-versa." This implies that Barry Allen is her "ex." But... when did that happen?
The only thing I can recall that even comes close is the brief moment in one of the early crossover episodes (can't remember which off the top of my head) when the two both acknowledge that they'd make a great couple, but that they can't because (in essence) she's a main character on a different show and she can't leave it to be with him on his show.
In other words, as far as I'm aware at least, he's someone where they both decided not to have a relationship in the first place. So what did Felicity mean with this line?