  The so-called grand canonical catastrophe of the density fluctuations in the
ideal Bose gas is shown to be a particular instance of the much more general
phenomenon of condensation of fluctuations, taking place in a large system, in
or out of equilibrium, when a single degree of freedom makes a macroscopic
contribution to the fluctuations of an extensive quantity. The pathological
character of the "catastrophe" is demystified by emphasizing the connection
between experimental conditions and statistical ensembles, as demonstrated by
the recent realization of photon condensation under grand canonical conditions.
