wow...I have so many. I'll start with other people's. I was in Grease and we were body miced and waiting for mic check. Our Doody had to use the abthroom and nobody knew so as she was being mic checked she was in the bathroom so we all heard her. She was washing her hands and talking (about the director lol).
Also, our Sonny never ever wore his pants during rehearsal and I was Marty so during the dance scene in rehearsals I had to dance with him in his boxers.
In rehearsals for lunch room/Summer Nights scene, Rizzo, Marty and Jan all enter with food trays. Being big chested and wearing a really loose shirt, I shoved our bottle of fake champagne into my shirt and nobody could tell. The three of us walk on and as the girls are supposed to be silent and just fake talking to each other, I pulled the bottle out of my shirt and we all passed it around.
When I was in Guys and Dolls the following year (a show i hated so much) I was Cartwright and had nothing to do until scene 6. I sat backstage and read a book and made phone calls.
Oh, another Grease one. One day our Danny was singing Sandy (we added it for him) in rehearsal and one of the company members with a great sense of humor got behind him and did this hysterical interpetive dance. Danny had no idea. At the end of the dance Danny gets back into the car and sighs. He got back in totally focused on his song and found the company member sitting in the passanger seat chin in hands mouthing Why-y-y-y? Oh Sandy. Danny stayed in character and looked at him and sighed at him and shook his head and kept a straight face. Of course the day of the show backstage we all did the dance.
Megan Mullally as Karen Walker on Will and Grace: "Tell me more. Tell me more. Like does he have a car?"