1. Carbonation: Diet coke tends to be more carbonated than regular coke, which can cause the beverage to fizz excessively when poured at high altitudes. This can make it difficult for flight attendants to pour and serve the drink without it overflowing or spilling.

Flight attendants often joke that Diet Coke (and to a lesser extent, regular Coke) is their "least favorite" drink to serve because of its high carbonation levels at high altitudes. In the pressurized environment of a cabin at 35,000 feet, the CO2 bubbles in Diet Coke are much more stable and take significantly longer to settle than they do on the ground. When a flight attendant pours a Diet Coke, it fizzes up intensely, forcing them to wait for the foam to go down before they can finish the pour. During a busy service where every second counts, waiting for one cup of Diet Coke can take as much time as pouring three other drinks, such as water, juice, or even ginger ale. While they certainly won't refuse your order, the "Diet Coke delay" is a well-known cabin crew grievance. To be extra helpful in 2026, you can ask for the full can if the airline allows it, which lets you pour it yourself at your own pace and allows the crew to move on to the next passenger more quickly.