Inspired by a gifset on Tumblr.
In Captain America: The Winter Soldier, Natasha keeps trying to set Steve up with various women, including Kate, his neighbor who's a nurse. Later in that same movie, Steve (and us the viewers) learn that Kate is really Sharon Carter and an undercover S.H.I.E.L.D. agent.
We don't see any direct interaction between Natasha and Sharon in CA:WS, but they are in the same room in Captain America: Civil War (the gif below). However, they never speak to each other in any of the films.
Do we know when Natasha learns about Sharon/Kate? Did she already know about Sharon's undercover mission and was trying to obliquely tip off Steve?