  We show that the errors due to atmospheric refraction are present in the
magnitudes determined with the Difference Images Analysis method. In case of
single, unblended stars the size of the effect agrees with the theoretical
prediction. But when the blending is strong, what is quite common in a dense
field, then the effect of atmospheric refraction can be strongly amplified to
the extend that some cases of apparently variable stars with largest amplitudes
of variations are solely due to refraction. We present a simple method of
correcting for this kind of errors.
