Tangency to a plane along a curve forces zero Gaussian curvature
= Tangency to a plane along a curve forces zero Gaussian curvature
After a rigid motion, write the plane as $z=0$ and the surface locally as $z=h(x,y)$. Along the curve of tangency, $h=0$ and $\nabla h=0$. Differentiating $\nabla h(\gamma(s))=0$ shows that the Hessian annihilates the nonzero tangent $\gamma'(s)$, so its determinant and therefore the Gaussian curvature vanish.