Equilibrium molecular dynamics is used to simulate fluids comprised of chains of tangent hard spheres. Reptation theory predictions of segmental motion are compared with simulation results. In addition to the usual tube confinement, a second entanglement effect is observed. As the chain disengages from the tube, persistent interchain contacts cause a plateau in the segment mean-square displacement and subsequent accelerated diffusion. Associated with the plateau in the mean-square displacement is a corresponding delay in relaxation of the end-to-end vector as interior chain segments are extended during disentanglement.