In The Prestige, Andy Serkis’ character has the following line:
Never spotted that. Nice 
I must be terribly dense. Can you help me out here?
Serkis played Gollum.
I don’t know if that was completely unintentional, though, since Serkis was already Gollum by then.
Dragon: The Bruce Lee Story has a mystical subplot about how an evil spirit is threatening him and then his son, only to be vanquished by Bruce in the end. Sadly, Brandon’s ultimate fate (on The Crow set) demonstrates that perhaps the evil spirit did ultimately emerge victorious. 
I kind of doubt that was unintentional, actually. It’s not like The Prestige was made before Serkis was cast as Gollum.
Edit - Blast, ninja’d by the tricksy hobbitses…
I get that, but how is it relevant? Because Bilbo hid the Ring in his pocket?
I am not nearly the LOTR fan that most folklorists are.
In Naked Gun 33 1/3, Frank Drebin retires from the police force and in his leaving speech says something like, “now, if I want to kill anyone, it will have to be in the privacy of my own home”.
Not that funny a line. OJ Simpson is in the assembled throng nodding along though…
In The Hobbit, Bilbo “wins” a riddle contest against Gollum when, feeling the Ring in his pocket, he absent-mindedly asks “What have I got in my pocket?” Gollum is unable to guess and has to show Bilbo how to excape from the caves he is lost in.
In 12 Monkeys, Bruce Willis says “Everyone I see is dead.” This is pretty close to the famous line in The Sixth Sense.
Of course, Kevin Costner in Field of Dreams says, basically, “I believe Oswald acted alone.” He later starred in JFK.
More creepy is the Groucho Marx line to Thelma Todd: “You’ll have to stay in the garage overnight.” Todd was later found dead in a garage – where she probably was overnight.
That would be Bull Durham, and I’m quite sure it was intentional.
What would be intentional about it? Bull Durham was made before JFK.
How about that? I guess I noticed it after JFK and assumed. Sorry, you’re right.
Not a movie, but there are a lot of jokes involving Phil Hartman’s character, Bill McNeal, that are just creepy and sad now. Gun jokes, death jokes . . . I can’t think of specific examples right now, but some of them really make me cringe.
Miss ya, Phil.
And then Gollum spends the next few decades asking himself “What has he got in his pocketses?”
Got it. Thanks.
Not a movie, but there was an episode of Seinfeld where Elaine was trying to get her boyfriend to change his name from Joel Rifkin (who was an infamous serial killer at the time). One of the names she suggested was O.J.
Also not a movie, but I always found it ironic how many prescient lyrics John Lennon sang with the Beatles. “Happiness Is A Warm Gun” being the prime example.
In Blazing Saddles, Gene Wilder’s character tells the story about how he gave up his gunslinging days when a young boy challenged him to a duel, and then shot him.
In Willy Wonka & the Chocolate Factory (made three years earlier), when Wilder’s character first meets Mike Teevee (wearing an old west gunslinger costume), TeeVee draws on him and pretends to shoot him.
I saw an episode recently where a psychic was visiting the station (part of the entertainment for Mr. James’s Halloween party). She told Bill’s future and predicted his death to be several decades away.
In that vein, there’s the famous SNL sketch from years back where John Belushi is visiting the graves of all the other original Not-Ready-For-Primetime players, and says that he’s the last surviving cast member from that group. In reality he was the first to die.