You may not be aware, but Mark Hamill's face actually remains the subject of a pretty longstanding debate. The actor suffered facial injuries in a car wreck between the filming of A New Hope and The Empire Strikes Back, and some fans believe the wampa attack scene (the Hoth Yeti) was written as a direct result of Mark Hamill's car accident. Carrie Fisher seemed to think so, but others involved in the film disagreed. Frustratingly, some behind-the-scenes photos build solid arguments for both sides.
So what really happened to Hamill's face? Did it always look like that, or was he hideously disfigured in a car crash to the point that filmmakers had to rewrite part of Empire's script? Dive into the mystery behind Mark Hamill's accident to try and determine whether or not it changed . . . anything at all.
Mark Hamill's car accident took place in January of '77, four months prior to the release of Star Wars. Given the timing, no one really knew who Mark Hamill was as he hadn't been in anything noteworthy to that point. In a 1978 Gossip magazine interview, Hamill admitted he thought his career was over after seeing his damaged face.
Fortunately for him, the movie that later acquired the contextually perfect name A New Hope proved to be a mega-success.
In the commentary on the Blu-ray of The Empire Strikes Back, George Lucas discusses Hamill's accident and his changed appearance in relation to the Wampa scene:
"My feeling was some time had passed, they've been in the Rebellion fighting, that kind of thing, so the change was justifiable. There's a scene in the film where Mark gets beat up by the [Wampa], which helps even more, but that wasn't really the meaning of why we wrote the monster in the beginning. We needed something to keep the film suspenseful at the beginning while the Empire is looking for them."
In a 1999 interview with Starlog, Hamill says he asked Lucas in private if the wampa scene was indeed meant to cover his facial damage, and the director told him it was not. While that's not definitive evidence, it is rather convincing as it doesn't seem there would be much motivation for Lucas to lie at that moment (unless he thought his star was a diva).
That being said, the fact that Hamill too questions the nature of the wampa scene shows even those involved in the production wondered about it.
In the same Blu-ray commentary, however, Fisher literally states the opposite:
"It was a really bad accident. Miraculously his teeth didn't shatter. But his nose did. He had to have some of his ear put into his nose. So they adjusted the film with this snow monster to right away in the movie scratch his face to account for his looks being different."