  As first realized by Witten an SU(2) gauge theory coupled to a single Weyl
fermion suffers from a global anomaly. This problem is addressed here in the
context of the recent developments on chiral gauge theories on the lattice. We
find Witten's anomaly manifests in the impossibility of defining globally a
fermion measure that reproduces the proper continuum limit. Moreover, following
Witten's original argument, we check numerically the crossing of the lowest
eigenvalues of Neuberger's operator along a path connecting two gauge fields
that differ by a topologically non-trivial gauge transformation.
