"common grammatical mistake" is correct. Adjective "grammatical" modifies "mistake" to create "grammatical mistake", then adjective "common" modifies "grammatical mistake". In "commonly grammatical mistake", "commonly" is modifying "grammatical".
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GPYIn "commonly grammatical mistake", "commonly" is modifying "grammatical". It's not very obvious what that would mean, and the phrase doesn't appear to make much sense.Of course, you could say something like "These mistakes are commonly grammatical mistakes", but in that case "commonly" modifies the verb, rather than forming a phrase "commonly grammatical m