  Sky temperature map of the cosmic microwave background (CMB) is one of the
premier probes of cosmology. To minimize instrumentally induced systematic
errors, CMB anisotropy experiments measure temperature differences across the
sky using paires of horn antennas with a fixed separation angle, temperature
maps are recovered from temperature differences obtained in sky survey through
a map-making procedure. The instrument noise, inhomogeneities of the sky
coverage and sky temperature inevitably produce statistical and systematical
errors in recovered temperature maps. We show in this paper that
observation-dependent noise and systematic temperature distortion contained in
released Wilkinson Microwave Anisotropy Probe (WMAP) CMB maps are remarkable.
These errors can contribute to large-scale anomalies detected in WMAP maps and
distort the angular power spectrum as well. It is needed to remake temperature
maps from original WMAP differential data with modified map-making procedure to
avoid observation-dependent noise and systematic distortion in recovered maps.
