  We discuss an instability in the leapfrog integration algorithm, widely used
in current Hybrid Monte Carlo (HMC) simulations of lattice QCD. We demonstrate
the instability in the simple harmonic oscillator (SHO) system where it is
manifest. We demonstrate the instability in HMC simulations of lattic QCD with
dynamical Wilson-Clover fermions and discuss implications for future
simulations of lattice QCD.
