"Is an aircraft's maximum gross weight a guideline from the manufacturer, or a number that should never be exceeded under any circumstances?"—anonymous
“The manufacturer has established a maximum gross weight for the airplane. This weight constitutes a limitation which must not be exceeded. It was established during testing by the manufacturer.
A pilot who exceeds this takeoff weight has no knowledge or assurance that the flight characteristics of the airplane will be acceptable. Additionally, FAR 91.9 specifically prohibits a person from operating an aircraft without complying with the operating limitations specified.
Information regarding the weight and balance for a particular airplane is located in the POH (Pilot’s Operating Handbook). The POH must be onboard the airplane where it can be used as a reference before a flight, and to determine whether the proposed loading of the airplane meets the limitations described in the weight and balance section of the POH.
In the case of light airplanes, it is sometimes necessary to operate with less than a full fuel load to meet the weight limitations specified for the airplane.”
Have you knowingly flown an airplane over it's maximum takeoff weight?