It's a common appellate refrain that you can't raise new arguments for the first time in your reply brief. But what counts as a new argument? For example, suppose you appeal and challenge a trial court ruling, explaining why it was wrong. Your opponent's brief then argues that any error was harmless. If, in your reply brief, you now explain why the ruling was not harmless, is that a new argument?
The answer is yes. On appeal, you must prove both halves of "harmful error" to prevail, which means your opening brief must explain both why the trial court was wrong, and what the harm was. Judge Prescott concurred in a recent Appellate Court case solely to remind practitioners of this point. The Court will occasionally overlook a lawyer's failure to comply. But that's an awful conversation to have with a client. Better to play it safe.
The case is State v. Carpenter, AC 41888. Judge Prescott's concurrence is available here: