https://en.wikipedia.org/wiki/Symbiosis_(Star_Trek:_The_Next_Generation)

"Symbiosis" was filmed after "Skin of Evil", which featured the death of Tasha Yar, making this the final filmed episode with Denise Crosby as Yar until she reappeared in "Yesterday's Enterprise". Towards the end of the episode, as Picard and Crusher leave the cargo bay, Crosby can be seen waving goodbye to the camera behind them (timestamp 42:15, she is clearly visible above Patrick Stewart's left shoulder as the bay doors close). LeVar Burton later used behind the scenes footage from this episode in a feature on his show Reading Rainbow, in the episode "The Bionic Bunny Show"