  We report on GROND observations of a 40 sec duration (rest-frame) optical
flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a
power law with indices +12 and -8, respectively, inconsistent with a reverse
shock and a factor 10$^5$ faster than variability caused by ISM interaction.
While optical flares have been seen in the past (e.g. GRB 990123, 041219B,
060111B and 080319B), for the first time, our observations not only resolve the
optical flare into sub-components, but also provide a spectral energy
distribution from the optical to the near-infrared once every minute. The delay
of the flare relative to the GRB, its spectral energy distribution as well as
the ratio of pulse widths suggest it to arise from residual collisions in GRB
outflows \cite{liw08}.If this interpretation is correct and can be supported by
more detailed modelling or observation in further GRBs, the delay measurement
provides an independent, determination of the Lorentz factor of the outflow.
