Widely considered to be one of the best episodes of Star Trek: The Next Generation's third season, "Yesterday's Enteprise" brought back Lt. Tasha Yar who'd been killed near the end of season one because Denise Crosby wanted to leave the series. Many fans thought she deserved a more noble death, and that's what this episode gave her.
Tasha Yar is alive and well aboard the Enterprise-C when it meets up with the Enterprise-D due to a rift in space time. On one side of the rift, there's a bloody Klingon War going on, and the end result of the episodes forces the out of time Enterprise to return through the rift which results in the deaths of all aboard, including Yar. The only person aboard the Enterprise-D to remember what actually happened was Guinan , who then asked Geordi La Forge to tell her more about Yar.
This excellent episode stands out among the 178 episodes of Star Trek: The Next Generation, and yet, Ronald D. Moore almost wishes the episode hadn't happened, according to Syfy.com. Instead, he wanted to use the premise for Star Trek: Generations, but it would have been Enterprise-A with Captain Kirk [William Shatner] at the helm coming through the wormhole.
"If we hadn't have done that episode, then [the movie] would have been the Enterprise-A coming through that wormhole, and you'd have Spock and Kirk and everyone on that ship, we'd play the same story. They — the original crew — they had to go back to their deaths. And Guinan knew Kirk, and Guinan knew Picard, and that would have been an amazing movie."
Essentially, the same story would have happened, only it would have been Kirk's crew heading back through the rift and knowingly to their deaths. So instead of just Captain Kirk dying in Generations as he did, his entire crew would have been killed off in the movie. Something tells me the fans wouldn't have liked that movie anymore than they did the original.