I just learned about this in a backstage tour last week. The actor playing Elphaba needs to step onto a very small platform in the darkness that has a safety feature that locks around her. There's a light that the stage manager can see that indicates when the actor is locked in. If the light hasn't lit up by a certain point, the stage manager will call for a "no-fly" version of the scene to take place instead of the flying version.
Possibly a problem with mechanical equipment, or was a unqualified understudy
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