Does the phrase "unusually common" actually make sense?
It's frequently used to imply something along the lines of, "It happens more than one would expect; it's common but you wouldn't think that it is." An example might be, "Contrary to popular belief, it is unusually common for pets to be allergic to their human owners."
But when you take a moment to analyze the lexical definition of the word "unusual", we find
not usual, [not] common, or [not] ordinary; uncommon in amount or degree; exceptional:
Putting the two together in a more literal fashion, it seems that "unusually common" translates to "not usually common", "not common common", or "not ordinarily common"; all three of which are the opposite of what the phrase itself is intended to mean. It seems like one of those phrases, similar to "I could care less", where there is an implied meaning that doesn't match the structure of the words.