TSQ, spot on! You can also experience this in a theater when you're distracted by the glow of a cell phone - and other artifacts like chatter, chair-kicking, etc. It's not a break of the 4th wall - technically correct, if anybody cares - but a break in the viewer's suspension of disbelief - with the same result. Whatever jars you from the hypnotic world of the imagination and reasserts your awareness of reality as a spectator watching a medium.
So an unexpected accident on-stage, or a prank... I used to work in musical theatre, and recall a few incidents.
We did
Peter Pan in the late '80s. The night that I decided to turn the props over to my assistant and watch the show from the audience, one of the pirates dropped his gun. It bounced on the stage, rolled over a couple of times, and fell into the orchestra pit. The audience found it tremendously amusing (they knew that wasn't supposed to happen), and I sat there cringing, hoping it wasn't broken (it was one of the props we'd borrowed). But the actor who played Captain Hook did his best to salvage the scene: "Good thing you dropped that gun; one shot and those Indians would've been on us!"
(I have since come to the conclusion that there are parts of Peter Pan that are revoltingly racist by modern standards)
So they had to do the rest of the first act without the gun, and I retrieved it at intermission to see how bad the situation was. It did need some repairs, and the actor was told to be very
very careful from then on.
A few years earlier, there was a prank that took some people out of the show. Again, this was a time when I was sitting in the audience for a night off (the show was
The Sound of Music). There's a scene in front of the curtain where a nun does a walk across while the stage crew is frantically putting the von Trapps' living room together behind the curtain.
This particular incident occurred on closing night, after the director sternly warned us not to play pranks, as this audience had paid to see the same show as all the others.
But apparently that warning didn't apply to himself. He borrowed a nun's habit and did the walk across himself. And yes, people noticed. One elderly lady elbowed her husband, and said, "Did you see that? That's a MAN!"

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