  Deriving physical parameters from gamma-ray burst afterglow observations
remains a challenge, even now, 20 years after the discovery of afterglows. The
main reason for the lack of progress is that the peak of the synchrotron
emission is in the sub-mm range, thus requiring radio observations in
conjunction with X-ray/optical/near-infrared data in order to measure the
corresponding spectral slopes and consequently remove the ambiguity wrt. slow
vs. fast cooling and the ordering of the characteristic frequencies.
  We observed GRB 151027B, the 1000th Swift-detected GRB, with GROND in the
optical-NIR, ALMA in the sub-millimeter, ATCA in the radio band, and combine
this with public Swift-XRT X-ray data.
  While some observations at crucial times only return upper limits or
surprising features, the fireball model is narrowly constrained by our data
set, and allows us to draw a consistent picture with a fully-determined
parameter set. Surprisingly, we find rapid, large-amplitude flux density
variations in the radio band which are extreme not only for GRBs, but generally
for any radio source. We interpret these as scintillation effects, though the
extreme nature requires either the scattering screen to be at much smaller
distance than usually assumed, multiple screens, or a combination of the two.
