Over weight luggage.
If you are about to fly on business or holiday, have your bags weighed on reliable scales before you leave home.
If the weight of you bags on the Airport scales show the bags to be heavier than when you weighed them at home, question the airlines about the accuracy of their scales.
I had a friend that had her bags weighed at the international airport in Hawaii. The bags came in at 18.6 kg and when the same bag, unopened was weighed on the domestic flight, they weighed in at 21.5. She questioned this and was told the scales were checked daily. She had to pay for the extra weight. When she got home she checked the weight of the bag (again unopened) on her digital scales and they came in at 18.6.
Always question the airport scales if it differs from the weight of your bag when you left home. Don’t let them bully you into accepting the accuracy of their scales.