Two problems related to mechanical jack have been reported for the 1999 Buick Century. The most recently reported issues are listed below.
The car had a flat and was being changed in a drive way on a flat surface with the jack that came with the car. Once the car was jacked up and the tire removed the jack broke and the front end fell down, no one was hurt, but someone changing the flat could very well have been hurt or killed when the car went down.
My wife tried to change a tire using the supplied jack which failed. The weight of the car crushed the jack which was on a flat asphalt surface. Clearly the supplied jack could not support the weight of the car.