I had this on LAX-NRT(!). Third pilot didn't show up, and the captain and purser (or whatever they are called these days) made it clear about their irritation. They didn't know when the replacement pilot could show up (traffic in LA)

I deplaned and went to the GA to get shifted to the soon-departing NH flight, but wouldn't keep my UG J seat to so do, since the flight was not yet in IRROPs(!?)

I talked to a great reservations agent who took 5+ minutes as I was walking toward TBIT who took the time to talk to operations (since they had not yet posted a delay) and communicated to me they had found a pilot and they would be arriving in 30 minutes, so no risk of cancellation. More info than the gate could get.

So, it happens. And UA treats it pretty seriously, especially for a intl flight.