The Good Student Who Writes the Bad Exam

I'm (finally) almost done grading my three (!) classes from last semester (note to self: that really wasn't the best idea you've ever had there, Steve). I like to think I would have finished yesterday if it weren't for my new best friend Eli "turning the corner" (for the 93rd time, which means we're back at 90 degrees). But I've already had one experience with the one class the grades for which I've submitted that I don't like: the problem of the good student who writes the bad exam.

My issue is never with what to do grade-wise. I only find out that it was the "good" student once the grades are in--and that's precisely why blind grading is such a good idea. (And for me, "good" student isn't just subjective; in Federal Courts, for example, the final was only 45% of the total grade, so I already had a basis for evaluating the students entering the exam). Rather, my issue is the inevitable conversation once we sit down to go over the exam -- "this is my worst grade in law school"; "I just don't know what I could've done differently"; "I had a bad day and don't know why"; etc. I'm not suggesting that these students are entitled to any different treatment than their peers. Quite to the contrary. At the same time, in my experience, the exam-reviewing conversation is just different in these cases (perhaps at least partially because these are the students who most frequently want to go over their exams in detail). In contrast to the conversation that's just about how a student's answers deviate from what was expected, the conversation with the good student is inevitably meta. Did the exam test the wrong material? Did the student prepare improperly? Did the student miss what the class was really about? Am I a sneaky bastard?

With each passing semester, I gain more and more confidence that their bad day is not my fault, and that I didn't write an unfair exam (which I care about much more than the degree of difficulty of each individual exam). But that doesn't help me shake the feeling that I've done something "wrong," or that, at the very least, I should treat the "B" (or even "C") student who bombed the exam after performing very well the rest of the semester at least somewhat differently from the way I'd treat the "B" or "C" student whose performance was consistent throughout.

But I'm curious for others' experiences... How do you handle this situation if/when it arises? And if you handle it differently, how do you do so?