To all those arguing that the EC nondiscrimination rule means that United MUST offer the same mistake fare to anyone in the EU that is offered to a Dane, and that the EU will certainly "educate" United about this if United says otherwise, a simple question.
Suppose the computer terminal at the ticketing counter at Barajas Airport in Madrid malfunctions, and as a result anyone who walks up to that ticket counter to purchase a ticket, for a week or so until the problem is fixed, ends up getting charged 1/3 of the regular fare.
United discovers the error and announces it will HONOR all the tickets as a customer service gesture.
At that point, someone who lives in Amsterdam brings an EU complaint arguing that United's customer service gesture violates the nondiscrimination rule, because the mistake fare was not offered on an equal basis to residents of Holland.
Do they win?
If your answer is "no", please explain how this case is different.
If your answer is "yes", I would respectfully say you are wrong. The issue of customer service gestures and dealing with the company's errors is simply not the same thing as discrimination (the discrimination rule would apply if United, for instance, published a base fare that was substantially different for residents of Holland than the published fare on the same flight for residents of Spain).