I have two, I guess...
My first was when I did "Wizard Of Oz" approximately a bajillion years ago. (Okay, it was six years ago, but it seems like a bajillion.) I was playing the Wicked Witch of the West, and it was such a small, low budget production that when I melted, I was supposed to lay under a black sheet onstage. I had to lay there while the cast danced around and sang "Ding-Dong, The Witch Is Dead!"
Well, I had on these fake fingers-- green and warty with long red talon-type nails. I thought it'd be funny if they were sticking out from under the sheet. But wouldn't you know, someone accidentally kicked my hand, and the fake fingers went FLYING into the audience. You can imagine there was all kinds of screaming and crying from the little kids in the audience after that.
My other, and MUCH more embarrassing one, was when I played Lydia Bennet in "Pride and Prejudice" about two years ago. Now, Lydia is, to put a fine point on it, a total harlot, with a good bit of bimbo mixed in. And it being "Pride and Prejudice," the whole cast was wearing these lovely low-necked, floor-length empire waist dresses that had elastic around the neckline and under the bust.
Well, there were about four dance numbers in the play, even though it wasn't a musical-- people went to balls all the time back then-- and in one of the dance numbers, my partner stepped on the hem of my dress. Which caused it to fall down, so I was naked from the waist up. Luckily, I was wearing a bra. Even more luckily, I was at the back of the stage, near the curtain, so I just went offstage and didn't come back on until I had a line. Surprisingly, no one in the audience that I talked to saw anything, probably because I was in the back, but several of the other kids onstage saw.
In my pants, she has burst like the music of angels, the light of the sun! --Marius Pantsmercy