Woody Allen and any film in which he romances a woman 40 or more years his junior.
Daryl Hannah as an astrophysicist in Roxanne, the worst of the 1000 or so recent movie roles in which a brainless beauty has been signed to play a Ph.D.
Jake Lloyd as Annakin Skywalker.
Hell, Jake Lloyd as ANYBODY.
I don’t know, Exapno, I kind of thought that was the point … she was beautiful (which Stud Fireman Guy could only see) yet she HAD brains, which Steve Martin appreciated. For the premise to work, the character HAD to be beautiful. I love that movie, though; Steve Martin is at his hysterical best. My favorite scene is when he’s chasing the old ladies, pretending to be an alien who prefers older female earthlings.
No, I thought Daryl Hannah was miscast in Wall Street, where she was briefly the love interest of Charlie Sheen. They had absolutely no chemistry and looked about as right as a couple as, oh, I don’t know, Boris and Natasha. 
Maybe that was the point, though, since she was only into him for his money.
Whoever that putz was in “Always” alongside Richard Dryfuss, John Goodman and Holly Hunter. Tom Hanks should have done that role.
Keanu Reeves in Twelfth Night. In the right role, the guy is very good. This wasn’t the right role.
I shall have to duel you to the death unless you retract your implied dis on his Hamlet. As the only Shakespearean (or Shakesperean-inspired) movie worth anything at all, I feel the need to defend it.
(If any of you know any Shakesperean films worth seeing, let me know.)
Sofia Coppola was absolutely dreadful in Godfather III
Denise Richards as nuclear weapon expert in The World is not enough
After you’re finished with him, then I get a crack at you for your implied dis on his Henry V…
Andie MacDowell. I have no idea why she gets movie roles.
Nice body
Rotten acting: "Maybe it’s her; something about MacDowell…is profoundly air-headed"
5 time champ I completely agree with you re: G3. That poor girl couldn’t act her way out of a paper bag.
Another Godfather miscast: Diane Keaton as Kay. Blech!
Lisa Kudrow didn’t work in Analyze This (and the sequel). She’s too young to be believable as Billy Crystal’s love interest; they looked ridiculous together.
Martin Sheen as Gen. Robert E. Lee in Gettysburg. WTF!
My vote goes to any actor over 50 paired with a love interest in her teens/twenties. Hello - Hollywood? Reality here. Women in their 40s and 50s and on are gorgeous, and seeing Grandpa Russell with Gwyneth had an extremely high yick factor.
I have not even seen the movie yet, but the trailer for that new Harrison Ford move, Hollywood Homicide looks awful. It looks like Eddie Murphy should be in his place.
I want to say that casting Charlton Heston as an (ethnically, mind you) Mexican cop in Touch of Evil was a stunning miscast, but somehow the movie ended up being flawless anyway.
Meryl Streep in “Postcards from the Edge.” I am willing to concede that she is a fine actress, but she just didn’t work in that role, perhaps because she was too old, perhaps because she had a little too much personal gravitas to convincingly portray a crack-addled child of privilege.
That would be “Yonda lies da castle of my fadda,” from The Black Shield of Falworth. 1954. I haven’t seen it myself, but it is infamous. (Oddly, it’s not listed in the IMDB’s quotes for the film.)
The whole cast of Dune (the eighties version).
**
Um, well, that doesn’t seem to be a mis-cast it seems to be right on target.
Marc
Ben Affleck in Sum Of All Fears. I know they were trying to make the character younger to keep the franchise going. But poor Bennie is such a lightweight I couldn’t buy him as a CIA analyst, action hero, or dramatic leading man in a movie involving a nuclear explosion. He was outclassed by everyone in this turkey, and the film still stunk.
Julia Roberts in Confessions Of A Dangerous Mind as the exotic femme fatale agent Patricia. Not exotic, not sexy, not dangerous. A lesser role, but she was also all wrong as the wife in Michael Collins.