I had a case interview today that was extremely difficult and went terribly. I went to [top school], but you would have thought I've never passed 8th grade math after the ridiculous problems I had.
I think there still might be a small chance of getting a 3rd Round, as I did all right during other parts of the day. I'm now constructing an email to the interviewer and was wondering if/how I should bring this up.
There's no point in sending an email to the interviewer to point out a mistake you think you made. 1) The decision has probably already been made, 2) if they didn't think it was a big deal, they will now, and 3) if they agreed you messed up, there's nothing you can say that will change their mind.
The decision was made the minute the interview ended. Sorry I don't have better news for you.
Also, I had one case at McKinsey final round that seemed awfully complicated. I didn't really solve the case because mostly I didn't even understand the case -- it was about health care insurance, which is absurdly complicated.
I think it was a poor case to use from an interviewer's point of view, because it required the interviewer to have pre-existing knowledge and I didn't.
I got the offer anyway because I nailed all the other cases (blew them out of the water), and I suspect others didn't understand the case I didn't do so well on either... bottom line, it's hard to predict what will happen, so try not to worry about it.