Sigmatropic reaction
In a thermal [1,3] hydride shift, a hydride moves three atoms.  The Woodward-Hoffmann rules dictate that it would proceed in an antarafacial shift.  Although such a shift is symmetry allowed, the Mobius topology required in the transition state prohibits such a shift. It is geometrically impossible. This is why enols do not isomerize without an acid or base catalyst.