Yes, I do encounter passengers who won't have a Diet Coke because they love Diet Pepsi, and occassionally such loyal folk seem to be really put out that we don't stock their cola of choice.

The funniest was a middle-aged lady who carried her own 6-pack of Tab in her large purse. I didn't even think they still made Tab and don't see it in my neck of the woods.
Brought back childhood memories of the 70's....