  The early optical emission of gamma-ray bursts gives an opportunity to
understand the central engine and first stages of these events. About 30\% of
GRBs present flares whose origin is still a subject of discussion. We present
optical photometry of GRB 180620A with the COATLI telescope and RATIR
instrument. COATLI started to observe from the end of prompt emission at
$T+39.3$~s and RATIR from $T+121.4$~s. We supplement the optical data with the
X-ray light curve from \emph{Swift}/XRT. %The optical and X-ray light curves
show very unusual behavior with features clearly beyond the standard fireball
model. We observe an optical flare from $T+110$ to $T+550$~s, with a temporal
index decay $\alpha_\mathrm{O,decay}=1.32\pm 0.01$, and a $\Delta t/t=1.63$,
which we interpret as the signature of a reverse shock component. After the
initial normal decay the light curves show a long plateau from $T+500$ to
$T+7800$~s both in X-rays and the optical before decaying again after an
achromatic jet break at $T+7800$~s. Fluctuations are seen during the plateau
phase in the optical. Adding to the complexity of GRB afterglows, the plateau
phase (typically associated with the coasting phase of the jet) is seen in this
object after the ``normal'' decay phase (emitted during the deceleration phase
of the jet) and the jet break phase occurs directly after the plateau. We
suggest that this sequence of events can be explained by a rapid deceleration
of the jet with $t_d\lesssim 40$ s due to the high density of the environment
($\approx 100$ cm$^{-3}$) followed by reactivation of the central engine which
causes the flare and powers the plateau phase.
