"Look upon the man who tells thee thy faults as if he told thee of a hidden treasure..." -- The Dhammapada (Chapter six)
This is probably one of the most difficult concepts to embrace in both martial training and in living life in general. That "man who tells thee thy faults," comes in many forms: a sensei, an uke, a friend, a spouse, and especially, one's self. If we're not open to looking at our own short-comings it could set us up for a fall. It could mean danger, for the spirit, or, on the street, physically.
Sometimes, while training, we express such flaws in technique in a jesting manner; other times it becomes a very serious conversation. As long as it solves the problem, the comments are considered a success.
As a friend, or an uke, I would be letting my partner down if I were to pretend a technique is working well if it is not. As a friend outside the dojo, the same applies.
But the hard part is always hearing such flaws about yourself.
As always, humility is a great teacher.