Why does one obvious GNSS error occurs sometimes?

Sometimes one obvious GNSS error occurs among all the reasonable GNSS fixes because of "static drift", an inherent feature of GNSS. It means when a GNSS terminal is stationary, its positioning is often changing, which can be caused by being under a roof, near tall buildings and walls, close to water surface or in a magnetic field. Such drift often happens during test. It happens much less in dynamics or after deployment.