  We investigate the performance of the hybrid Monte Carlo algorithm, the
standard algorithm used for lattice QCD simulations involving fermions, in
updating non-trivial global topological structures. We find that the hybrid
Monte Carlo algorithm has serious problems decorrelating the global topological
charge at the values of $\beta$ and $m$ currently simulated, where continuum
physics should be approximately realized. This represents a warning which must
be seriously considered when simulating full QCD by hybrid Monte Carlo.
