Grammatical error or grammar error?

Which of the 2 is corect? "Grammatical error" or "grammer error"?
Update: Obviously, I mispelled "grammar" :-)
Update 2: "a noun does not describe another noun. So the answer to your question is grammatical error."

What about "system error"? :P
But I know what you mean :) To say "grammar error" could imply that there is an error with the entire grammar system.
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