Far too many people pulling a fast one by leaving their carry ons out of sight of the check-in staff. Some airlines like AF are pretty strict about hand luggage...if you don't get a label at check-in for it then it won't be allowed on.

Most problem occur from passengers who are connecting from long haul on to short haul. The sizes of carry on bags I've seen is ridiculous...business travellers being the worst offenders I've noticed.