The death of R. Lee Ermey got me to thinking about how “Full Metal Jacket” is really two movies in one. The first is a basic training movie, the second is a Vietnam war movie. Neither half really seems to have much to do with the other.
Oddly enough, the other example that popped into my head was “Stripes”, which was also a basic training movie combined with a post-basic-training plot that seemed like another movie entirely.
From Dusk Till Dawn: Hard-boiled crime drama for the first half, psychedelic monster party for the second.
“Life is Beautiful” is the first one that pops into my head. The first half is about a guy setting up his life (pursuing his bride to be) in a new village. The second half is about their family’s experience in a Holocaust camp.
Oh, what about Psycho? Would that count, what with the decoy protagonist?
Here’s an oldie, though you might catch it on TCM now and again. The 1940 film They Drive by Night is ostensibly about long haul truck drivers.
The first half of the film is about two brothers (George Raft and Humphrey Bogart) starting their own independent trucking business, and the various challenges involved in running a business like that.
The second half suddenly transforms into a noirish crime drama, as the wife of a rival trucking business owner murders her husband and tries to seduce the George Raft character, then frames him for the murder after he rejects her. Most of the stuff about trucking goes out the window while Raft tries desperately to prove his innocence.
Movie Movie, of course. A boxing film and a musical. 
Jonathan Demme’s Something Wild (1986): Starts as a wacky romantic comedy about straight-laced yuppie Jeff Daniels getting his boring life turned upside down by Melanie Griffith. It turns into a completely different, very tense movie when psycho ex-husband Ray Liotta shows up.
How about The World’s End?
Starts off as a movie about a bunch of high school friends getting together to catch up and quickly changes theme at the mid point. I had no idea about the twist in the plot before seeing it in the theatre. My wife went WTF? and didn’t enjoy the rest of the film.
LooKing for Richard alternates between a documentary about making a film of Shakespeare’s Richard III, and and actual performance of Richard III. In my opinion, one of Al Pacino’s more interesting films.
Or Barton Fink, which starts out as a kind of satire of Hollywood and scriptwriting, but turns into … something else…
Society almost feels like this.
First hour is a below average forgettable creepy mystery. The last half-hour is one of the most horrifying sequence you’ll find out there. Full-blown vomit inducing.
Screaming Mad George is one weird dude. He did the effects. :eek:
Check it out. 
Definitely. I still think it’s the best example of pulling the rug out from under us. I was lucky enough to see it in my teens not knowing the secret(I know!). It blew me away.
When the chair is turned around and is a skeleton at the end, I gasped.
I can not imagine the audiences in 1959.
Stranger Than Fiction is the love story of a baker and an IRS agent and also the IRS agent realizing his life is being narrated by Emma Thompson for her book.
Julie and Julia goes back and forth between an autobiographical portrait of Julia Childs, which was brilliant, and a modern-day depiction of a young woman’s attempt to cook everything from Julia’s original cookbook. This second movie was pretty ho-hum compared to the first one.
Grindhouse was so much two movies that they sold the DVDs of “Death Proof” and “Planet Terror” separately.
The 1984 comedy Best Defense was in effect two movies. The poster even sort of looked like two movies – a Dudley Moore film and an Eddie Murphy film
Butch Cassidy and the Sundance Kid kinda fits the bill. The first half is their robberies and chase in the American west, the second half is their increasingly desperate time in Bolivia. Like Full Metal Jacket, the first half is better.
The Caine Mutiny starts as shipboard military film and becomes a courtroom drama.
And I’d almost include The Princess Bride. The beginning where Vizzini and crew kidnap Buttercup and Westley chases after them is distinct from the intrigue and machinations that come later.
The Caine Mutiny is the first one that came to my mind. 90 minutes of naval war film, then 30 minutes of courtroom and legal drama. One of my favorites.
2001 of course. The first time I saw it I thought I had rented the wrong movie. I was expecting science fiction, but for the first 20 minutes it’s a nature documentary.
Edit: Damn - scooped on the Caine Mutiny while I was typing!
I suppose Richard Lester’s The Three Musketeers is cheating? 
If “anthology” films count, then you could also include Trilogy of Terror from 1975 and the various Twilight Zone movies.
Not quite the clean break of some of the movies above, but The Happening is two decent ideas for movies crammed into one terrible movie. One is a horror movie above a disease that causes people to kill themselves. The other is an apocalyptic movie about a plague where the only way to survive is to split into increasingly smaller groups until you are completely alone.
Jaws is a “Friday the 13th” film with an invisible killer fish, followed by a three-man “Moby Dick” pastiche.