Coke is nice. It doesn't matter if you have it to quench your thirst or you mix it up with Old Monk. Although, there might be a slight issue when you order it on a plane. If you are wondering that you could die by doing so, then no. But you will be slowing down services inside the plane and might cause slight inconvenience to other passengers and staff.

Source: Mirror

Well, according to a report by the Daily Mail, the pressure inside the cabin makes cold drinks to foam up a lot more than they normally do. And among these cold drinks, the Diet Coke causes the worst problem. The report quotes flight attendants saying that they could serve three other passengers in the amount of time taken to pour a can of diet coke. One of the flight attendants revealed in her blog, "These Gold Wings" that the aircraft cabin is not pressured to sea level but to the equivalent of about 7,000 or 8,000 ft.

Source: Guru Focus

Which means, that traveling passengers could feel light headed and one could say that the effects of alcohol will be almost twice as it would, on sea level. She said that the same circumstances make soft drinks to foam a lot more when they are poured out of a can. Flight attendants claim that Diet Coke takes up the most amount of time to pour.

Source: Business Insider

Although, flight attendants claim that they do not resent any passenger that orders Diet Coke, but let's be honest here. Why be a dick, when you can avoid it? It's easier to choose another drink to go with your flight.