Bill Murray accidentally agreed to be the voice of Garfield in 2004. It's one of his biggest regrets.

The story is a pretty funny one. Bill Murray is an actor pretty well known for being very specific about which roles he picks. He’s starred most notably in films like Caddyshack, Groundhog Day, Ghostbusters, and of course Zombieland, more recently. 

He’s so discerning about roles that he doesn’t even have an agent. It took a lot of people by surprise, then, that he was the voice of Garfield in the critically panned film. It might seem strange that an actor so specific about what roles he takes would make the mistake of signing on to a film being written by someone completely different from who he thought.

The Garfield writer was named Joel Cohen, which Murray mistook for Joel Coen of the Coen Brothers. The Coen Brothers are responsible for multiple universally acclaimed films like No Country For Old Men, O Brother, Where Art Thou?, and The Big Lebowski, a movie so influential it spawned its own religion. 

So it stands to reason that a movie about Garfield the cat done by the Coen Brothers would be nothing short of genius. It wasn’t until the line recording was well underway that Murray realized it was a different Joel Cohen. 

Though the story explains his line in Zombieland where he says he regrets doing Garfield, it still doesn’t explain why he signed on for the sequel in 2006. You can check out the interview where he goes into detail about it at the source

(Source)