  In order to constrain the broad-band spectral energy distribution of the
afterglow of GRB 100621A, dedicated observations were performed in the
optical/near-infrared with the 7-channel "Gamma-Ray Burst Optical and
Near-infrared Detector" (GROND) at the 2.2m MPG/ESO telescope, in the
sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio
frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an
interpretation of the observational data within the fireball scenario.
  The afterglow of GRB 100621A shows a very complex temporal as well as
spectral evolution. We identify three different emission components, the most
spectacular one causing a sudden intensity jump about one hour after the prompt
emission. The spectrum of this component is much steeper than the canonical
afterglow. We interpret this component using the prescription of Vlasis et al.
(2011) for a two-shell collision after the first shell has been decelerated by
the circumburst medium. We use the fireball scenario to derive constraints on
the microphysical parameters of the first shell. Long-term energy injection
into a narrow jet seems to provide an adequate description. Another noteworthy
result is the large ($A_V$ = 3.6 mag) line-of-sight host extinction of the
afterglow in an otherwise extremely blue host galaxy.
