I hope no company YC makes this mistake, considering How to Win Friends and Influence People [0] is recommended reading in YC [1].

For example: Don't criticize, condemn, or complain. and Let the other person save face.

That is, criticize in private, give praise in public (give the person a fine reputation to live up to). Calling someone a retard is a counter-productive thing to do.

0: https://en.wikipedia.org/wiki/How_to_Win_Friends_and_Influen...

1: http://news.ycombinator.com/item?id=5584 (can't find where I read it, but this thread points to pg recommending it)