Harmonic and anharmonic quasiclassical barrier samplings were used in classical dynamics simulations to evaluate product energy distributions (PEDs). The results obtained for the CH dissociation in the methanethiol cation (CH3SH+ --> CH2SH+ + H) show that the PEDs are changed under inclusion of anharmonicity in the initial conditions. Both the vibrational energy content at the transition state and the energy in the transitional modes are important to explain the differences found in the PEDs. Discrepancies between the PEDs obtained for trajectories initiated at the barrier and those initiated at the reactant were found and explained on the basis of dynamical phase space bottlenecks, which make the phase space density not uniform for t not equal 0.