  The critical behaviour of three-dimensional semi-infinite Ising ferromagnets
at planar surfaces with (i) random surface-bond disorder or (ii) a terrace of
monatomic height and macroscopic size is considered. The
Griffiths-Kelly-Sherman correlation inequalities are shown to impose
constraints on the order-parameter density at the surface, which yield upper
and lower bounds for the surface critical exponent $\beta_1$. If the surface
bonds do not exceed the threshold for supercritical enhancement of the pure
system, these bounds force $\beta_1$ to take the value $\beta_1^{ord}$ of the
latter system's ordinary transition. This explains the robustness of
$\beta_1^{ord}$ to such surface imperfections observed in recent Monte Carlo
simulations.
