Its very unusual. That's why they have reserve flight crew. I've even seen reserve crew flown between hubs (RDU-JFK and DFW-MIA) to cover flights. In fact I've had a flight from MIA-SCL delayed by 2.5 hours in order to wait for reserve crew to arrive in MIA from DFW for our flight.

That said, if they are out of bodies then there is not much they can do. One problem that I can see is that it screws up the crew rest time. I've been on another MIA-SCL flight that had a second set of flight crew sitting in J class to cover the return of SCL-DFW when a flight was canceled.