In the first season episode "The Naked Now", Tahsa Yar and Data have an (ahem) intimate moment. Although Tahsa expresses her desire to forget it at the end of that episode, other episodes refer to this special relationship between these characters ("Skin of Evil", "The Measure of a Man", "The Most Toys").
In the episode "Yesterday's Enterprise", Tasha reappears in an alternate timeline. Clearly this means that her death in "Skin of Evil" never happened. But did they still have the intimate affair of "The Naked Now"?
Quotes by her daughter Sela, novelizations, and expanded-universe materials are fair game.
Not really related: