Just to throw out an anecdote: a while back I nearly was denied boarding of an international flight, because it turned out the airline had mixed up my passport data and decided I was not a US citizen and would need a visa to return to the United States.

I discovered this at the airport, when trying to drop off my bag. Fortunately, the airline agent was able to get a supervisor who took a look at my (US citizen) passport and fixed something in their records that allowed me to get a boarding pass and catch the flight, but I could easily have just been stranded there by someone who took the attitude of "computer says no, can't help you when the computer says no".