Silence of the Lambs. Everyone knows what it’s about now, but they wouldn’t if all they had to go from was its title.
Similarly, Cat On A Hot Tin Roof. When I watched that, I was worried it would be some obscure symbolic reference that I wouldn’t understand, but then she came right out and said it.
OTOH, if “Cat on a hot tin roof” is or was a common southern saying, then it might make more sense, but of Tennessee Williams coined it for the play I’m sure a lot of people were confused.
The Postman Always Rings Twice
No, people usually called him the Don. You can check the script. Vito Corleone is only called the Godfather a handful of times and most of them are in the first few minutes during the wedding scene when people are asking him for favors.
Trainspotting
I think a lot of people are still unclear about that. I remember after watching it asking a friend what it meant and he said “Ummm, maybe because they, um, watched trains and smoked pot?”
I just pulled up the wiki page for the movie which doesn’t mention anything, but the wiki page for the book says: “‘Trainspotting’ is a slang term for injecting heroin: the drug running along the ‘tracks’ or veins. It is also said that ‘trainspotting’ is slang for spotting the drugdealer, or being on the look out for a drugdealer.[SIC]”
The title conjures impressions of cheap erotic thrillers starring Shannon Tweed and Eric Roberts - instead of the globe trotting espionage film that it is - 2008 Ridley Scott-directed spy thriller Body of Lies.
The tacky title might be a red flag to the over-the-top action sequences and convoluted difficult-to-follow plot that is to come. I managed to have fun with it though - despite the misleading title, overly-important explosions, cars barreling through crowded Middle Eastern market bazaars, all the grenade launching, ginsu-knife tossing, sky-track surveillance shots constantly reminding everyone to marvel in awe at the telescopic reach of CIA’s big brother eye, the cold war overtones, one after another, reminiscent of decades-ago USSR paranoia - it was a pretty good film. 
)
The Breakfast Club
The Deer Hunter
Minority Report
The Iceman Cometh had nothing to do with George Gervin
I remember leaving the theatre after seeing “Johnny Got His Gun” and some people in front of me were wondering about the film title since the main character is not named Johnny. Being a buttinsky, I told them it was because during World War I there was a popular song called “Johnny Get Your Gun”. Which isn’t true, it’s actually the opening lines in George M Cohan’s “Over There”
When Ralph Bashki's "Fritz the Cat" came out, a theater where some friends worked had a mother and father drop off three 10-12 year olds, thinking it was a children's film. The manager had to call the parents and tell them it was an X rated cartoon.I can think of at least two things wrong with that title.
Krakatoa, East of Java (Krakatoa is west of Java).
The lurid title Panic in the Streets doesn’t fit the movie’s story (a doctor and a detective succeed in keeping the public in the dark about a possible plague outbreak until after they have eliminated the risk).
Watch on the Rhine takes place in Mexico and Washington, D.C., thousands of miles from the Rhine.
The 49th Parallel is named for the line that forms most of the U.S.-Canada border but the only border crossing depicted in the film is at Niagara Falls, which is much farther south.
Jezebel is not a biblical epic.
Hud is not about Housing and Urban Development.
D.W. Griffith’s Intolerance attempted to tell four separate stories, only one (arguably two) of which had much of anything to do with intolerance.
Concerning Blade Runner it turns out that the title (and nothing else about the movie) was borrowed from a novel about a black-market dealer in medical supplies (including scalpels, presumably).
Then there are a bunch of 1980s movies set in the South that I always get confused because they have such nondescriptive titles that are all touchy-feely noun phrases Places in the Heart
Tender Mercies
Terms of Endearment
Also a bunch of 1980s movies about youths, kids, and/or teachers with less-than-descriptive titles all in the imperative mood that I get confused
Stand by Me
Lean on Me
Stand and Deliver
Do the Right Thing
Seeing some of the examples here, I have to wonder what sort of titles would be considered honest and clear.
Blade Runner is not about a roller blader, perhaps one on the run from the law (gimme a break, I grew up in the late 80’s early 90’s when such a movie would make perfect sense).
Gorky Park ultimately has very little to do with the park in Moscow, but then, that was more because Martin Cruz-Smith, who wrote the book the movie was based on, had a deep love of multiple-reference puns (see also, the books Polar Star, Havana Bay, Red Square, and pretty much everything else he has written). Typically the title makes perfect sense once you start doing the downhill run at the end of the book, but the setup always involves some minor thing sharing the same name that ultimately ends up not as important as it first seems.
Starship Troopers is not an adaptation of the Robert A. Heinlen novel about power-armored paratroopers defending humanity. 
(the latest movie, Starship Troopers: Invasion, BTW, is much closer in tone to the original book, although the plot is not from said book).
???
Oh, that’s right. Music did not exist before YouTube. All those R.E.M. albums (CDs) we bought and enjoyed in the mid-1990s? It was all just a dream. Just a dream…
The Adventures of Robin Hood
Around the World in 80 Days
Back to School
Batman Begins
The Bridge on the River Kwai
Clerks
The Creature From the Black Lagoon
Deep Throat
Driving Miss Daisy
Finding Nemo
Forrest Gump
Gandhi
Grand Hotel
Gunfight at the OK Corral
Hot Tub Time Machine
How the West Was Won
Indiana Jones and the Temple of Doom
Iron Man
Kill Bill
King Kong vs Godzilla
The King’s Speech
Kramer vs. Kramer
The Life of Emile Zola
Marty
Massacre at Central High
Mutiny on the Bounty
Patton
Sink the Bismarck
Snakes on a Plane
Titanic
Toy Story
Willy Wonka and the Chocolate Factory
The Wizard of Oz
Yellow Submarine
Hal Holbrook was great in that.
As I kid, I remember being quite excited when I saw there was a movie titled The Effect of Gamma Rays on Man-in-the-Moon Marigolds that night on TV.
I kept thinking “Soooo… when is the hulk appearing, to fight the irradiated plants?”
Because there was a lot of freaking country there. And old men! Actually only the old men were left by the end of that movie and they were out in the country and there was lots of it for the two of them!
As a title The Road does not let the potential viewer know that it is the most depressing movie ever!
I’m seeing two classes of unclear titles here. One type is only unclear if you lack a piece of background information. The Godfather, The Man in the Moon, Michael Collins and The Last King of Scotland are good examples, in order of increasing obscurity. The Godfather may have been a somewhat unclear title when it was first released, but due to cultural osmosis it’s now as obvious as Gandhi. The second type conveys no meaningful information about the film, such as Michael Clayton, North by Northwest, Blue Velvet, and Cloverfield.
Has anyone come up with an example of a deceptive title yet, which was the other thing the OP asked for? I’m not seeing one, but I haven’t seen all the films mentioned.