This chapter describes the most common sources of uncertainty affecting calibrated NICMOS data. Some of these will be propagated into the Error Array of the calibrated data (e.g., flatfield uncertainties) or into the Data Quality Array (e.g., grot and hot pixels). These sources all represent limiting factors for the sensitivity of the NICMOS observations. In addition, some error sources found during the Science Mission On-orbit Verification (SMOV) are described (e.g., non-zero zeroth read, dark current pedestal), to help observers decide whether their data need to be recalibrated or analyzed with particular care.