  We have analyzed the Chandra observation of the distant (z=0.164) galaxy
cluster RXJ1720.1+2638 in which we find sharp features in the X-ray surface
brightness on opposite sides of the X-ray peak: an edge at about 250 h_50^{-1}
kpc to the South-East and a plateau at about 130 h_50^{-1} kpc to the
North-West. The surface brightness edge and the plateau can be modeled as a gas
density discontinuity (jump) and a slope change (break). The temperature
profiles suggest that the jump and the break are the boundaries of a central,
group-size (d=380h_50^{-1} kpc), dense, cold (T=4 keV) gas cloud, embedded in a
diffuse hot (T=10 keV) intracluster medium. The density jump and the
temperature change across the discontinuity are similar to the ``cold fronts''
discovered by Chandra in A2142 and A3667, and suggest subsonic motion of this
central gas cloud with respect to the cluster itself. The most natural
explanation is that we are observing a merger in the very last stage before the
cluster becomes fully relaxed. However, the data are also consistent with an
alternative scenario in which RXJ1720.1+2638 is the result of the collapse of
two co-located density perturbations, the first a group-scale perturbation
collapse followed by a second cluster-scale perturbation collapse that
surrounded, but did not destroy, the first one. We also show that, because of
the core motion, the total mass inside the cluster core, derived under the
assumption of hydrostatic equilibrium, may underestimate the true cluster mass.
If widespread, such motion may partially explain the discrepancy between X-ray
and the strong lensing mass determinations found in some clusters.
