  In the internal shock scenario for GRBs we expect some fraction of the energy
of the burst to be carried by slow moving shells that were ejected at late
times. These slow shells collide with faster moving outer shells when the outer
shells have slowed down as a result of sweeping up material from the ISM. This
gives rise to a forward shock that moves into the outer shell producing a bump
in the afterglow light curve of amplitude roughly proportional to the ratio of
the energy in the inner and the outer shells. In addition, a reverse shock
propagates in the inner shell and produces emission at a characteristic
frequency that is typically much smaller than the peak of the emission from the
outer shell by a factor of $\sim 7 \gamma_{0c}^2 (E_2/E_1)^{1.1}$, and the
observed flux at this frequency from the reverse shock is larger compared to
the flux from the outer shell by a factor of $\sim 8 (\gamma_{0c}
E_2/E_1)^{5/3} $; where $\gamma_{0c}$ is the bulk Lorentz factor of the outer
shell at the time of collision, and $E_1 & E_2$ are the total energy in the
outer and the inner shells respectively. The Lorentz factor is related to the
observer time as $\sim 5 (t/day)^{3/8}$. The shell collision could produce
initial temporal variability in the early afterglow signal. The lack of
significant deviation from a power-law decline of the optical afterglow from
half a dozen bursts suggests that $E_2/E_1$ is small. Future multi-wavelength
observations should be able to either detect bumps in the light curve
corresponding to both the forward and the reverse shocks or further constrain
the late time release of energy in ejecta with small Lorentz factor, which is
expected generically in the internal shock models for the gamma-ray bursts.
