  We study the uncertainty in galaxy cluster mass estimates derived from X-ray
data assuming hydrostatic equilibrium (HE) for the intra cluster gas. Using a
Monte-Carlo procedure we generate a general class of mass models allowing very
massive clusters. We then compute the corresponding temperature profiles via
the HE equation and compare them to observational data on some clusters. We
find several massive clusters that pass the observational constraints, with
integrated masses varying in a quite wide range. The resulting accuracy of the
mass estimates is rather poor, larger than what is generally claimed. We argue
that the tight constraints on cluster masses previously obtained come from the
fact that a too restricted class of mass density profiles has been investigated
so far. Applying our procedure to Perseus then Coma, we find that the
improvement of the observational constraints results in a quite modest
improvement in the accuracy of the mass estimate. For Coma, using the best
current available data, we end up with a factor of two of uncertainty in the
mass within the Abell radius. This uncertainty rapidly increases at further
radius.
