Denise Crosby left the character of Tasha Yar in the first season of Star Trek: The Next Generation. Though her character was killed off in "Skin of Evil," the previous episode "Symbiosis" was actually filmed afterwards, making that Crosby's final episode.
In a "blink and you'll miss it moment," as Captain Picard [Patrick Stewart] and Dr. Beverly Crusher [Gates McFadden] are leaving the cargo bay after sending the Ornarans and Brekkans back to their planets along with the felicium, you can see Crosby waving in the background. It's at 42:21 in the episode on Paramount+. As it turns out, it wasn't scripted.
An eagle-eyed fan asked about the wave in the December/January 1998 edition of Star Trek Communicator, the Magazine of the Official Star Trek Fan Club, wondering who Crosby was waving to, and if the action was planned. The response was that as this was, at the time, Crosby's last shot on the series, she decided to wave goodbye to the camera.
Because there were no issues with the scene, only one take was filmed. No other footage was available for the editors to use so they used what they had, perhaps hoping viewers wouldn't notice. it really is such a quick moment that you have to be paying attention to see it. As this fan obviously was.
Of course, this didn't turn out to be Crosby's last shot in front of the camera on Star Trek: The Next Generation as she returned in season three's "Yesterday's Enterprise" as an alternate reality version of Lt. Tasha Yar. Crosby returned again in seasons four and five, playing Commander Sela, the half-Romulan daughter of the alternate timeline's Yar and a Romulan general.
But, for those of you who did notice the wave, now you know what it's about, and for those of us who didn't until this bit of information became available, well, now you can go look for it. I know I did!