I believe this behavior is unique to 2016.
When I go to place a mate and SW does not like it (over defining the assembly) I get the warning box. That is fine, but.
It does not matter which button you select in the box, the mate is places anyway and you must go back and delete it manually.
The proper behavior is that when the NO box is selected the mate should not be placed.
This causes extra work.
Why would I want to place a failed mate?