  The discovery by Swift that a good fraction of Gamma Ray Bursts (GRBs) have a
slowly decaying X-ray afterglow phase led to the suggestion that energy
injection into the blast wave takes place several hundred seconds after the
burst. This implies that right after the burst the kinetic energy of the blast
wave was very low and in turn the efficiency of production of $\gamma$-rays
during the burst was extremely high, rendering the internal shocks model
unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and
show that the efficiency of converting the kinetic energy into gamma-rays is
moderate and does not challenge the standard internal shock model. We also
examine several models, including in particular energy injection, suggested to
interpret this slow decay phase. We show that with proper parameters, all these
models give rise to a slow decline lasting several hours. However, even those
models that fit all X-ray observations, and in particular the energy injection
model, cannot account self-consistently for both the X-ray and the optical
afterglows of well monitored GRBs such as GRB 050319 and GRB 050401. We
speculate about a possible alternative resolution of this puzzle.
