I don't understand the fascination that certain posters have with even pointing out that had the OP been better prepared, this might not have happened. Everyone is agreeing that it's United's fault, and very few people think that anyone should be required to bring a printout of visa rules just in case.

Just because you would have been more forceful in preserving your trip against United's incompetence doesn't mean that should be the base expectation for all travelers. It's easy to be in a bubble here on Flyertalk and think all travelers should be as prepared for any contingency as you would, but come on already.