Several optical errors present in interferometer systems are examined in detail. Optical cavity errors are typically the primary limitation on measurement accuracy. Secondary sources of error include imaging distortion and ray-mapping (slope) errors. The effect of phase modulation/processing errors on precision is briefly discussed. Some on- site errors (operator technique and environmental effects) are also examined. Rules of thumb for error estimation are presented when applicable. Specific quantitative analysis is based on the Zygo MARK IVxp phase measuring interferometer.