  In this letter we present results from intra-night monitoring in three colors
(BRI) of the blazar S4 0954+65 during its recent (2015 February) unprecedented
high state. We find violent variations on very short time scales, reaching
magnitude change levels of 0.1-0.2 mag/h. On some occasions, changes of ~0.1
mag are observed even within ~10 min. During the night of 14.02.2015 an
exponential drop of ~0.7 magnitudes is detected for about 5 hours.
Cross-correlation between the light curves does not reveal any detectable
wavelength-dependent time delays, larger than ~5 min. Color changes
"bluer-when-brighter" are observed on longer time scales. Possible variability
mechanisms to explain the observations are discussed and a preference to the
geometrical one is given.
