  We study zero-range processes which are known to exhibit a condensation
transition, where above a critical density a non-zero fraction of all particles
accumulates on a single lattice site. This phenomenon has been a subject of
recent research interest and is well understood in the thermodynamic limit. The
system shows large finite size effects, and we observe a switching between
metastable fluid and condensed phases close to the critical point, in contrast
to the continuous limiting behaviour of relevant observables. We describe the
leading order finite size effects and establish a discontinuity near
criticality in a rigorous scaling limit. We also characterise the metastable
phases using a current matching argument and an extension of the fluid phase to
supercritical densities. This constitutes an interesting example where the
thermodynamic limit fails to capture essential parts of the dynamics, which are
particularly relevant in applications with moderate system sizes such as
traffic flow or granular clustering.
