  Immediately after they are born, neutron stars are characterized by an
entropy per baryon of order unity and by the presence of trapped neutrinos. If
the only hadrons in the star are nucleons, these effects slightly reduce the
maximum mass relative to cold, catalyzed matter. However, if stangeness-bearing
hyperons, a kaon condensate, or quarks are also present, these effects result
in an increase in the maximum mass of up to $\sim 0.3{\rm M}_{\odot}$ compared
to that of a cold, neutrino-free star. This makes a sufficiently massive
proto-neutron star metastable, so that after a delay of 10--100 seconds, the
PNS collapses into a black hole. Such an event might be straightforward to
observe as an abrupt cessation of neutrinos when the instability is triggered.
