What likely happened in this case is they gave GP a burger intended for someone who ordered it 'plain'. The slips McDonalds uses to attach special order tags to the sandwich wrappers are prone to falling off or being overlooked.

N'ah, the employees were just overworked so made mistakes or cut corners. The amount of customers that come at an airport or a train station at rush hour is insane. Mistakes happen, or some employees cut corners to save time, knowing that the customers won't have time to notice the mistake and complain about it, since they have a train/flight to catch.