  We investigate the performance of the hybrid Monte Carlo algorithm in
updating non-trivial global topological structures. We find that the hybrid
Monte Carlo algorithm has serious problems decorrelating the global topological
charge. This represents a warning which must be seriously considered when
simulating full QCD, regardless of the number and type of fermions, with this
or any similar algorithm. Simulated tempering is examined as a means of
accelerating the decorrelation.
