The temperature resolution of infrared focal plane arrays is limited by temporal and spatial noise. The spatial noise usually is partially removed by correction procedures. These correction procedures reduce the spatial noise to a magnitude below the temporal noise. The correctability c defined as the ratio of the spatial to the temporal noise is a figure of merit for the state of the correction. We consider the transient degradation of the correctability after correction. A new figure of merit, the long-term stability time constant (tau) lts is introduced. This time indicates the duration after a nonuniformity correction during which the spatial noise increases to values higher than that of the temporal noise. Several staring infrared focal plane arrays differing in size and in detector material are investigated. The correctability c is determined after various correction procedures and the long- term stability time (tau) lts is measured. The degradation of the correctability is caused by a few individual pixels in the detector array. We can classify three different types of `bad pixels,' which degrade the correctability. These are weak pixels that show a low responsivity and flickering and drifting pixels that show excessive 1/f-noise.