Rapid temperature quenches have long been known to produce metastable thermodynamic phases. We analyze a Landau-Ginzburg model of front propagation and show that metastable phases can arise via a transition that splits the front separating the disordered from the stable phase into one front between the disordered and the metastable phases and another between the metastable and the stable phases. For systems described by a single nonconserved order parameter, the splitting transition is continuous with no hysteresis as the control parameter is varied. For two order parameters, the transition can be continuous, hysteretic, or require a finite-amplitude perturbation. We also discuss briefly applications to pattern-forming systems, where the pattern formed behind a propagating front may change discontinuously as the front velocity is increased.