  We re-explore the symmetries of a weakly isolated horizon (WIH) from the
perspective of freedom in the choice of intrinsic data. The supertranslations
are realized as additional symmetries. Further, it is shown that all smooth
vector fields tangent to the cross-sections are Hamiltonian. We show that
joining two WIHs which differ in these Hamiltonians and boundary data, under
the action of a supertranslation, necessarily require the inclusion of an
intermediate dynamical phase, possibly with the inclusion of a stress energy
tensor. This phase of the boundary is non-expanding but not a WIH and
invariably leads to a violation of the dominant energy condition. The
assumptions made allow us to reconstruct the (classically) pathological stress
energy tensor also.
