They don't charge for breathing.

Most of the problems occur before you get on the plane and are caused mainly by people who think rules don't apply to them or didn't bother to read the rules in the first place.

So when they say hand luggage 55x40x20cm they mean it has to fit into a box that size so you need to check for anything that sticks out like handles or wheels.
When they say one piece of hand luggage they mean one piece of hand luggage, not one piece plus handbag plus rucksack plus newspapers plus meal deal plus the duty "free".
When they say 10kg hand luggage they mean 10kg - not 10.01kg.
When they say 15 or 20kg of checked luggage they mean exactly that.