On falling:
In perhaps the most underrated comedic performance of all time, Phillip Seymour Hoffman is introduced into the movie Along Came Polly by stepping onto a waxed floor in mid-greeting and hitting aforementioned floor HARD!
It should be in the mental repository of anyone into falls.
Also, Elf getting hit by the cab in Elf gets funnier every time I see it.
Okay, now all I need to know is what a luvvie is! Is it a thespian? Not that there’s anything wrong with that. 
Oh my God, me too. I have an acquaintance that is famous for this in our circles – “It’s raining sheets of dogs.” “Don’t bite the gift horse.” Stuff like that.
Relatedly, when someone accidentally says the wrong word entirely, it cracks my shit right up. This one is harder to explain, but I’m sure you’ve all seen and/or done it. The brain misfires and somehow a completely irrelevant word shows up in a sentence rather than the one intended. My favorite example of this was when I was on a walk with a friend of mine. (This was in high school, so there’s some not-PC verbiage coming).
My dog was with us, and my friend kicked a tennis ball for him, but his foot just glanced over the top of the ball and sent it rolling gently into the shrubbery three feet away rather than rocketing it down the street the way he’d intended. And he said, “I should be a kicker for a deflated volleyball team. Oh my God. I mean, I should be a kicker for a retarded soccer team.” HA HA HA HA! It was literally years before I could tell that story without collapsing in laughter halfway through… and when I did get through it, no one thought it was as funny as me. Oh well.
If blown performances are funny, and falling is funny, then I guess a guy who is exposed lip-synching when he falls off the stage should he hilarious:
People trying not to laugh and mostly failing kills me. Example
Dramatic Chipmunk is the funniest thing ever. Or possibly Sneezing Panda.
Bwahahahahaha!!!
One thing that often cracks me up is love scenes in novels where the author is trying to show that something really hot and sexy is happening, and yet he uses euphemisms for absolutely everything.
One that I read recently that I remember was a Guy Gavriel Kay book (Lions of Al-Rassan) where there is a scene where one character basically has a night-long fuck-fest with someone he just met, complete with bondage and leaning-over-the-railing-doggie-style on a balcony one floor up from a massive carnival celebration. And yet, the author chooses words like “her urgency” or “his manhood”. It just somehow makes the whole thing less sexy and makes me laugh instead.
Another is one I read just last night. “Shadow Prey” by John Sandford (not a book I’d recommend, but I have nothing else to read at the moment!). So fucking funny, I’ll just quote it directly.
Seriously? Her trousers?? Her underpants?? THE hot liquid center?? It’s not even hers?
I burst into laughter when I read it, and read it aloud to my husband. His response:
“What is she, a lava cake?”
I am still giggling like an idiot over that one!
Darn sure had me chuckling.
The YouTube of the commedian that brought three men on stage, then couldn’t continue his act because he was giggling too hard at the older gentleman’s laugh just slayed me and I think it would fall into several of the categories already mentioned by posters here. That one made me laugh for minutes on end. So yes, in general the “corpsing”.
Also, people’s reactions when they’re startled can be pretty amusing. AFHV had the clip of the guy who thinks his friend is being electrocuted and he’s running in place screaming while the friend pretends to zap himself off a light switch.
Gotta second this one. When my 3 year old was exactly the wrong height and smashed her head into my groin when running, I was curled up on the floor, simultaneously moaning from the pain and laughing at the humor of it.
Lederhosen Fall Down
Anybody who disagrees with Revenant on this one should watch the infamous ‘Stephen Colbert eats a banana’ thing from the Daily Show a few years back. It might be funnier than any joke they got right.
After seeing my girlfriend’s show over the weekend, I realized that people pretending to do ballet is always funny, as long as they are pretending to be good at it. It’s gleefully silly stuff.
I think it’s funniest when somebody puts on airs of importance, acts all high brow and cultured, then makes an inappropriate comment clear out of the blue, whether it be cursing, sexual, politically incorrect, or TMI.
I remember when two elderly southern gentlemen at my workplace were talking about area golf courses, their games, clubs, etc, then started talking about one course in particular. One of them then spoke in a hushed whisper, “I heard they started letting JEWS in there.” I immediately burst out laughing like a madman and they looked at me like I was.
We’ve done this thread before.
Clearly, the funniest thing ever is:
Richard Simmons on Whose Line Is It Anyway?
I love Greg Proops at the end. “I’m just so happy!”
This is, for me, the funniest commercial ever. Everyone has their own sense of humor, and for whatever reason, this hits mine right in the sweet spot. It is absolute comic perfection. Every time I think about it, I giggle irrepressibly.
It’s for “fusionone.com” and it came out maybe 4-6 years ago.
Lame synopsis for lame people who don’t want to watch it:A man is sitting on a park bench eating lunch. Suddenly, two paramedics run up to him and start performing CPR, applying pressure to his shoulder, and shouting, “Find the limb! I need the limb!”
Meanwhile, the guy is confused, but after a few frenzied seconds, he fights the paramedics off, saying “Guys! Guys! I’m fine! Look! I’m fine!”
They exchange puzzled glances, quickly apologize, and leave.
As the man settles back down to his lunch, we hear an engine roar to life, and suddenly from the bushes behind him bursts a chimpanzee wielding a chainsaw above its head. As it runs, screaming, at our unfortunate hero, he desperately looks back after the just-departed medics.
Cut to corporate graphic, and voiceover: “Sync is everything.”
My favorite part of the whole thing is the expression on his face right at the end!
Also: ape + chainsaw = hilarious!!
One previous poster mentioned laughing at bad puns while another mentioned laughing at fart jokes. Personally, I’d much rather hear the worst pun in the world than the best fart joke. (Any 3-yr old can tell great fart jokes, folks!) I guess puns make people think too hard. Actually, any thinking is too hard for people esp. nowadays, huh? (Information age? We don’t need no stinking information age! 21st Century and we’re still in the Dark Ages! 
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Along these lines, I watched “The Jerk” on TVLand for the first time, and it confirmed my gut instinct when it first debuted. Luckily, people like Steve Martin and George Carlin eventually evolved from the primordial muck into (relatively) refined humor. I don’t even mind good slapstick, but crude humor is so amateur…anyone could do it. Steve Martin always had a good act inside him (without the vulgarity); he just had to find it in his own time. Pitty it took him so long!
If the early days of radio and TV hadn’t been so rigid, we wouldn’t have had the genius of people like Groucho Marx who just had to be suggestive, not a potty mouth. Even Benny Hill knew he just had to paint a picture in our dirty little minds. Rent some old comedy and see the difference! (Yes, Mr. Wisenheimer, the golden years of radio and TV were before my time, too, I’ll have you know! 
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For me the funniest thing ever was when I made a friend of mine laugh so hard that he shot a chili bean out of his nose.
I also think it’s pretty funny when people laugh so hard they snort.
Agree with the OP. I once saw Jason Alexander lose it laughing during The Producers onstage because of something Martin Short was doing; Alexander was unable to continue with the scene until he recovered. The audience was laughing as hard as he was, and this went on for several minutes.
One time my little brother fell through the floor. Our parents had bought a fixer-upper type house and it was under heavy construction. He was walking toward me and fell through some boards. Ended up in the basement. I think I laughed for three days.