My question in the 'serious misunderstanding' realm would be whether this is a problem that this individual student is having, or is it a common misunderstanding across the class? In the latter case, I turn the question back out to the class, eg, "Who thinks that's the right answer, who thinks it's Something Else?" And then we demonstrate the the individual student isn't alone in the Serious Misunderstanding, and provide a bit of personal buffer between the response and the correction.

I want students to come away with a self-description of "I have lots of ideas, and they need to be carefully checked because sometimes they're wrong," as opposed to the incredibly prevalent "I'm bad at math."