In a 747-400 with 340 passengers it would be a completely normal flight to have at least a couple of passengers faint during a flight. The subjects are predominantly female and faint after staying in their seats for too long before getting up quickly to go to the toilet. The moment they stand up all the blood leaves the brain and they faint. If you are a smoker you are already walking around at sea level as if you are at 5000' so adding some extra altitude to that gives you instant hypoxia. People with circulatory problems like diabetes or varicose veins seem to be particularly at risk.
Sitting them down on oxygen for a few minutes normally fixes the problem.