It was terrifying enough for me in the gig I worked on that dealt with reservations and check-in, where a catastrophic failure would be someone boarding a flight when they shouldn't have. To avoid that sort of failure, the system mostly just gave up and issued the passenger what's called an "Airport Service Document": effectively a record that shows the passenger as having a seat on the flight, but unable to check-in. This allows the passenger to go to the airport and talk to an agent at the check-in desk. At that point, yes, a person gets involved, and a good agent can usually work out the problem and get the passenger on their flight, but of course that takes time.

If you've ever been a the airline desk waiting to check-in and an agent spends 10 minutes working with a passenger (passengers), it's because they got an ASD and the agent has to screw around directly in the the user-hostile SABRE interface to fix the reservation.

SABRE is pretty good compared to the card file it replaced.

It's better to say SABRE replicated, in digital form, that card file. And even today the legacy of that card form defines SABRE and all the wrappers and gateways to it.