You can understand how this can happen. You have a computer and everyones' boarding card. You also have all the luggage which is scanned. No one should have a bag who has not boarded the flight. You compare one list to the other using the computer before you take off. It's simple security isn't it?
Well no. Air France seem to have mucked this up. On 13th October, their AF990A flight from Paris to Johannesburg had to turn back to France after 45 minutes when someone on board finally compared the luggage list with the passenger list and discovered that there were 4 bags on board without passengers. It appears the bags had been transferred from Heathrow on a connecting flight to Paris but 2 of the passengers had missed the connection but their 4 bags had made the flight.
As a result of the mix up, several hundred passengers suffered a significant flight delay, arriving in Johannesburg over 3 hours late.
Perhaps Air France will ensure their staff check their records more closely in future.