Factual error: When the President's jet is on final approach, they pass a helicopter carrying a donkey. The helicopter is able to maintain position alongside for some time. In fact, when it peels away it still doesn't fall back, but just gets farther away.
Welcome to Mooseport (2004)
1 factual error - chronological order
Directed by: Donald Petrie
Starring: Gene Hackman, Marcia Gay Harden, Ray Romano, Maura Tierney
Continuity mistake: When Bob and Handy are talking beside the Moose at the start of the film, they are talking about Handy's nose. Right after Handy says that when Bruce (The Moose) is there, his nose looks normal. As he says this, Bob looks down. Then as the camera changes angle to look at Bob for the reply, he is once again looking directly at Handy.
Trivia: Monroe Cole states that people will be calling him Mr. President for the rest of his life. While it is true people do tend to call former presidents by that title (thus not a movie mistake), as a point of trivia; is it incorrect to address a former president as "Mr. President" or "former president (surname)." While Cole doesn't have to worry about his ex-wife taking his title, leaving office will do that. It's the media that perpetuates this presidential title error.
Question: Why did Gene Hackman quit acting after appearing in this movie?
Answer: As the other fine answers indicated, it was multiple reasons. An additional one was his health. Hackman said his doctor told him his heart wasn't healthy enough to withstand the stress and physical rigors involved in filming movies.
Moreover, Hackman didn't get along with the director, Donald Petrie, and stated that he hated this movie.
Answer: It was said, that he got tired of doing movies where everything and everyone explodes and dies a bloody death.
Answer: His decision to retire wasn't related to this film; it just happened to be his last. He was working on other projects (mostly writing), and wanted to focus on those. He didn't "officially" announce his retirement from acting until 2008.