  We consider a pair of unstable fermions in a spin-entangled state. After the
decay of one fermion, a spin measurement is performed on the surviving partner,
with a Stern-Gerlach experiment or similar. The measurement not only projects
the spin of the surviving fermion, but is also physically equivalent to a spin
projection for the decayed one -- even when it no longer exists. This
post-selection effect would be experimentally accessible using muon pairs in a
maximally-entangled state, produced either in the decay of a scalar particle,
or in $e^+ e^-$ collisions at wide angles.
