I saw a show get stopped once.
It was the first performance I saw of "Wicked."
I don't know if the scene is still there...or if the Emerald City guard still has the line. But Fiyero was in charge of a group of men in search of Elphaba, who had found Dr. Dillamond in a cell in the Wizard's palace. Doctor Dillamond was a goat, and was the last animal faculty member on the staff of Shiz University until he mysteriously disappeared. By the time Elphaba found him, the doctor had lost the power of speech and could only "Baaa-a-a-a-a."
Elphaba spirited him off the stage as the guards approached. Suddenly, Fiyero arrived on the scene, and one of the guards said, "Sir, there's a goat on the lam."
Stopped the show cold.
Doubt seriously they left it in. But it certainly stunned the actors on the stage!