George Lucas made a little movie back in 1976. You may have heard of it. Star Wars was unwatchable to the few filmmakers who got to see it. But it was an important project for Lucas, one he'd already poured his best efforts into. So they went back and re-edited the entire thing. When I say "they," I mean the production team, but the magic that took place in the re-edit was due to the talents of Marcia Lucas, George's wife at the time.
David Welsh of RocketJump Film School goes through how the rough cut was changed into what we now know as Star Wars: A New Hope. You'll see a lot of the rough footage that was either deleted or changed -and even more important, why it was changed. You can see why Marcia Lucas won an Oscar for editing the movie (along with Paul Hirsch and Richard Chew). -via Metafilter