  The exponential sensitivity of cluster number counts to the properties of the
dark energy implies a comparable sensitivity to not only the mean but also the
actual_distribution_ of an observable mass proxy given the true cluster mass.
For example a 25% scatter in mass can provide a ~50% change in the number
counts at z~2 for the upcoming SPT survey. Uncertainty in the scatter of this
amount would degrade dark energy constraints to uninteresting levels. Given the
shape of the actual mass function, the properties of the distribution may be
internally monitored by the shape of the_observable_ mass function. An
arbitrary evolution of the scatter of a mass-independent Gaussian distribution
may be self-calibrated to allow a measurement of the dark energy equation of
state of Delta w ~0.1. External constraints on the mass_variance_ of the
distribution that are more accurate than Delta var < 0.01 at z~1 can further
improve constraints by up to a factor of 2. More generally, cluster counts and
their sample variance measured as a function of the observable provide internal
consistency checks on the assumed form of the observable-mass distribution that
will protect against misinterpretation of the dark energy constraints.
