Grammatical mistake
I believe the answer is:
solecism
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grammatical mistake</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for solecism that I've seen before include "Social gaffe" , "Incongruity" , "Absurdity" , "A mistake in the use of language" , "Absurd or mistaken word or act" .)