TNG, Season 1 - “Five Yar Mission” - Whilst reading up on the five-year format that exploration missions used to take back in the 23rd century, Picard spots an amusing typo in the book, claiming Kirk was assigned to a “5 Yar Mission”. However, Q chooses this moment to show up and have some fun of his own by putting the crew on a literal version of that typo, creating four duplicates of Tasha Yar, placing each of them, as well as the original Tasha in seperate cells on the brig, and tasking the senior staff to deduce which Tasha is the original. In the end, Riker deduces that the original Tasha is back in Q’s “penalty box”, and that the fifth Tasha in the Brig was also one of Q’s creations.
The episode was cancelled, as by this point, “Skin of Evil” was in pre-production” and it was deemed unsatisfying to have so much tension over Tasha’s continued existence only to then kill her off anyway so soon after.