Apparent violations of the no-slip boundary condition are studied using a series of narrow molecular-weight distribution polybutadiene melts (67300 less than or equal to (M) over bar(n) less than or equal to 650000), subjected to plane-Couette shearing over clean silica glass surfaces. Simultaneous measurements of slip velocity and shear stress reveal several new molecular characteristics of slip in entangled polymers. log-log plots of slip velocity versus shear stress display three distinct power-law regimes: (i) A weak slip regime at low shear stresses that is characterized by extrapolation/slip lengths b of the order of a few micrometers; (ii) A stick-slip regime at intermediate shear stresses marked by periodic oscillations in slip velocity and shear stress; (iii) A strong slip regime beyond a defined critical shear stress sigma*. Slip violations in this last regime are characterized by large slip velocities and massive extrapolation lengths (b(infinity) similar to 100-1500 mu m). For all polymers studied the critical stress sigma* for the weak-to-strong slip transition is found to be proportional to the plateau modulus G(e) of the bulk polymer, sigma* approximate to (0.2 +/- 0.02) G(e). This finding is consistent with a shear-induced polymer disentanglement explanation for apparent slip violations in entangled polymers. Our experimental observations are also found to be in good agreement with a recently proposed scaling theory for friction and slip in entangled polymers, which assumes noninteracting surface chains. We rationalize this last result in terms of a polymer adsorption model in which a single macromolecule spontaneously attaches to numerous surface sites, yet offers a sufficiently long tail to resist relative motion of a chemically identical bulk polymer that attempts to slide over it.