@ACuriousMind I will appeal to your wisdom. I've just read something... unique (in case I'm not clear enough, I'm talking about eq. (48-49)
here). The author describes, what is usually known as the unitarity gauge, as
not being a gauge transformation (for reference, it looks a little different than the usual Higgs lagrangian because he is neglecting fluctuations of the modulus of the order parameter)