US'ns have essentially forgotten that data is plural. Well, some folk use the plural verb with it but that is sounding increasingly pedantic. Media is becoming increasingly common in computer contexts as a straight singular, e.g. "Insert non-write protected media". Agenda is commonly used to mean a list of items to be covered in a meeting or suchlike and is invariably construed as singular, as in "I have an agenda here for tomorrow's meeting". The most common use for opera is a collection of lyrics and music in the form of some drama or comedy, and is taken as a singular. The fact that it derives from the singular opus has long since been forgotten.