Legal writers should always aim for grammatically correct prose. Why? To me, the key reason is to establish and maintain your credibility: When readers see that you know how to write correctly, they are also likely to believe that you stated the facts and law correctly and presented accurate, valid arguments, explanations, or advice.
But occasionally, grammar itself becomes an issue in a case. Here are some grammatical matters that made it into caselaw, including one that made headlines.
Although using the passive voice is not a grammar error, using it unwisely can have bad consequences. In one case, a due-process claim written in the passive voice failed to identify who did the “assuring” in the following statement from the complaint: “she was assured that she would continue on the job as long as she performed satisfactorily.” As a result, the judge dismissed that claim.