  Variability amplitudes larger than 1 magnitude over time-scales of a few tens
of minutes have recently been reported in the optical light-curves of several
blazars. In order to independently verify the real occurrence of such extremely
violent events, we undertook an observational study of a selected sample of
three blazars: PKS 0048-097, PKS 0754+100, and PKS 1510-089. Possible
systematic error sources during data acquisition and reduction were carefully
evaluated. We indeed found flux variability at intra-night time-scales in all
three sources, although no extremely violent behaviour, as reported by other
authors, was detected. We show that an incorrect choice of the stars used for
differential photometry will, under fairly normal conditions, lead to spurious
variability with large amplitudes on short time-scales. Wrong results of this
kind can be avoided with the use of simple error-control techniques.
