What about the Flyers fan diving into the penalty box and getting pummelled by Tie Domi? 
Or very funny. {Sorry, Leaffan. Just kidding.}
I think I’ve already mentioned when Pippin was singing the song while Faramir and his men were getting slaughtered and Denethor was eating his lunch I snickered because it looked to me like Pippin was crying over Denethor’s messy eating habits.
I bumbled into The Tin Drum late one night around 1990. I came in during a Nazi rally and thought it was a documentary at first, but it became clear this was stuffily self-important mess was one of the greatest unintentional comedies of all time. The horse’s head, the post office assault, the oversexed kid, the midget porn, the way the subtitles kept using “somnambulist” instead of “clairvoyant” (she does this ONSTAGE??), it was all gold. Highly recommended.
And no one has mentioned Reefer Madness? I’m certain that was not intended to be funny when created but is a laugh-riot in the midnight showing at the local theaters. The wild-eyed, manic-looking guy smoking a joint? The “car-speeding-crazily” and the cut to the speedo showing 55mph?
Ow, oww, stop it! Your making my sides hurt!!
Hahaha! Oh man, now I’ll never be able to take this scene seriously again.
sniff …all over that fine table cloth 
Try watching Excalibur for the first time, after you’ve got Holy Grail committed to memory. You’ll have to put a sock in your mouth.
The part in The Fellowship of the Ring where Galadriel tempts Frodo is one of the most unintentionally hilarious things I’ve ever seen on screen. It’s so embarrassingly STUPID and overwrought, and just seems like a bad 1950’s B-movie moment. I burst into laughter in the theater.
E.T., I fell on the floor laughing in the movie theater when E.T. ‘died’.
In the remake of The Champ, I burst into laughter when Jon Voight buys it and the kid turns on the waterworks.
I’m not totally heartless, Old Yeller and Where the Red Fern Grows will have me in tears every time. I think it is something about dogs.
Towards the end of [b}Aliens**, just about every scene featuring severed torso-Bishop. It was decidedly unsettling to look at him, and yet I couldn’t stop laughing. What a combo.
Oh, and in Constantine, the scene where that guy with the spear walks across a field full of cows and the cows drop dead as he approaches. Heh.
We made fun of Kinsey so much, my friend came out of it thinking it was supposed to be a comedy.
Yes. That was funny!
Al Pacino’s Scarface .
Exhibit 1: “You cockaroacha!”
Exhibit 2: " The world is like a giant puuuusssy waiting to be fahhhked !
Exhibit 3: "Ju want to fahhhk me Tony? Ju want to fahhhk your seester?
Exhibit 4: Pacino’s head coming up from the giant pile of coke on his desk.
For me it wasn’t so much funny as just realy weird and wooden voice acting. But yeah, that is the only thing I remember about that flick.
One thing I’ve posted before is from “The Day The Earth Stood Still.” The army doctors are speculating why the 90-year-old alien is so youthful and healthy. One of them posits that Klaatu’s folks have super advanced medical science while simultaneously offering the other doctor a cigarette. Gee, maybe they don’t smoke? First of all, it’s completely unintentional. The actors were just doing to have something to do with their hands while speaking. The fact that it’s Doctors makes funnier. And of course the humor of hindsight, like listening to people in the '60’s make fun of Ronald Reagans political ambitions.
Speaking of Reagan, doctors and smoking, I saw a movie once (I’m not sure of its name) in which Jane Wyman plays a lady who’s elated to learn she’s pregnant. To help her celebrate, the doctor offers her a cigarette.
Way back when it was a first run Movie, My 13 year old friends and I laughed our heads off at the exploding and melting Nazis in Raiders of the Lost Ark
The guy going into the propeller of the flying wing inspired the death of many a grasshopper into a cox .049 engine powered spinning blade of insect death.
Old movie smoking scenes were parodied brilliantly by an old SNL sketch starring Phil Hartman as Trent Markham, Lung Doctor:
…and of course I screw up my Paste and miss the most relevant part:
When she said, “All men shall worship me and despair!” and then turned green , I was waiting for the Doctor Who end credits to roll: Diddly dang, Diddly dang, WOO WOO! WOO WOO!
You know, we could have a whole thread of really bad LotR moments, and yet taken as a whole they were such good movies. Weird.
Glad I could do that for you
Case Sensitive I think it is partly because taken altogether in the EE format they are so freakin’ LONG there are bound to be some bad moments. Especially if they are watched obsessively over and over as some of us are wont to do.