Symmetry principles and theorems are of crucial importance in optics. Indeed, from one side they allow to get direct insights into phenomena by eliminating unphysical interpretations; from the other side, they guide the designer of photonic components by narrowing down the parameter space of design variables. In this Letter we illustrate a significant departure from the Babinet spectral complementarity in a very common and technologically relevant situation: that of a patterned conducting screen placed on a subwavelength dielectric slab. The symmetry property predicted by Babinet theorem is correctly recovered for pairs of geometrically complementary - but less realistic in terms of applications - free-standing patterned screens. Our analysis merges experimental data with fully vectorial electromagnetic modeling, and provides also an alternative form of Babinet theorem that highlights a connection with the concept of electromagnetic duality.
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