Had something similar happen at the ending of a production of Brighton Beach Memoirs. When Stanley (the older brother) gives Eugene (the younger brother) a picture of a naked lady at the end, the actor playing Stanley produced what is still one of the filthiest pieces of pornography I’ve ever seen.
The actor playing Eugene pretty much lost his shit, even though he told me he was expecting it.
This might be one of those “you had to be there” stories, but I’ll give it a go…
Went to see The Notebook with my mom and sis. (We always go for the cheesy girly flicks when we leave the menfolk at home.) Completely packed theater…we got there fifteen minutes early and still ended up sitting in the very back row over to the side.
So we get towards the end of the movie, and you can tell the audience is really into it because there’s total silence as we move into the scene where the elderly versions of Noah and Allie climb in bed to die together. As we see the discovery the next morning, the tension in the theater is palpable…and out from the silence comes this very loud, very tragic, sobbing “Oh. My. GAWD!” Some lady towards the front of the theater has just totally lost it.
I look over at Mom and Sis, and they’re looking back at me, and we simultaneously lose it ourselves…only in the opposite direction. Only problem is, we’re the only ones in the audience who seem to find it amusing…everyone else is still silently watching the very sad, romantic ending to the film.
Oh, my god, the pain. I couldn’t breathe. Trying so hard not to ruin the end for everyone else, but every few seconds a little snort escapes. And then I think I’ve got it under control, but another snort or gasp comes from my Mom or Sister and we’re off again. I remember glancing at the guy sitting next to me and he’s watching us with this confused look on his face, and I thought we must look like we’re having simultaneous epileptic fits or something. And that just made me want to laugh even more.
Thank god there were only a couple of minutes left in the movie…the moment the credits rolled the three of us let loose. I don’t think I’ve ever laughed so hard. To this day, all one of us has to say is “Oh. My. Gawd” and we’re off again.
Like I said, you probably had to be there, but it’s one of my favorite movie-going memories. 
In college, a guy had written a short musical which was kind of a parody of Franz Kafka’s The Trial. The show called for some audience participation: during the first scene, a cop would roam the audience and “arrest” a random spectator and drag them upon the stage, where the cast would sing a funny song about all the crimes they were charged with. Heinous criminal acts such as stepping on cracks (and therefore breaking your mother’s back), wearing sweatpants outdoors in a non-exercise capacity, crotch-stuffing, accessory to aggravated wearing of a brown belt with black trousers, and so on.
At the end of all of this, the whole chorus crescendos, “How do you pleeeeeeeeeeeead?” And a reporter jams a live microphone in the chump’s face so the audience can hear his answer.
Now, at this point, every audience member selected said something like “not guilty” or “innocent” or “I didn’t do it” or “I was framed!” If they were feeling snarky, they might say “guilty” or “temporary insanity!” But that was OK, because the writer had come up with scripted responses to every conceivable plea, and the actors were good improvisers in case somebody said something unexpected.
On the second-to-last performance, they go through the whole ritual, and everybody sings, “How do you pleeeeeeeead?” and the reporter sticks the microphone in the guy’s face. He leans into the mic and issues forth a truly monstrous belch. It was magnificent.
Luckily, the audience busted a gut for a good two minutes, which gave the actor playing the judge plenty of time to improvise his next line. “Uh, I’ll enter that as Not Guilty!”
We did Musical Comedy Murders of 1940 in high school, and there was one girl who a bunch of us had a thing for acting in it. One of us was particularly… odd, and was known to take dares as personal affronts. One random night when it was dead, the subject of flashing came up, and cute girl would not believe that our friend would actually flash anybody in public.
Fast forward to closing night. A handful of us had figured out a way to get the friend off duty for five minutes during the show so he could “change.” At one point in the show, cute girl’s character peers into a hidden doorway in a bookcase.
And there was our friend, wearing nothing but socks and a sign that said, “And you thought I wouldn’t do it; shame on you.”