Well, this is the unlucky case of the old names described in the genus Musca (in the sense of the old authors, i. e. belonging to many diverse families and genera today).

When Panzer in 1798 described his tachinid Musca libatrix, there was already a Musca libatrix existent, named by Scopoli in 1763, but this Musca libatrix of Scopoli is a Syrphidae (according to the Catalogue of Palaearctic Diptera its valid name is Sphaerophoria scripta Linnaeus, 1758). So Musca libatrix of Panzer is a primary homonym of Musca libatrix Scopoli, and when strictly applying to Art. 57.2 Panzer's junior name would be invalid. This was the reason why Chandler changed the name to fulva Fall?n, a mere formalism and not helpful in my opinion. It is clear, however, that Zenillia libatrix fulfills the condition of Art. 57.2.1 (which refers to Art. 23.9). So Zenillia libatrix remains valid because it is a nomen protectum (you may find the full text of the ICZN articles on http://www.iczn.o.../index.jsp).

There was never a problem with the use of the two names. Nobody ever confounded libatrix Scopoli and libatrix Panzer because the species are of course quite different and systematically widely separated, and even the names could not have been confounded because libatrix Scopoli was not used as a valid syrphid name. Art. 23.9 was introduced to the Code to avoid such unnecessary changes of names.