Isn't everybody a bit of these at some points in time?
I'm pretty sure asking for references from provided references and contacting them without getting permission from the applicant is illegal. Also most people would feel there was a serious invasion of privacy I think.
Not sure how it is in any way illegal.
Certainly it can violate any kind of implied discreteness in the search. I would think that you'd need permission, but then it might not work as well. The best you could say is, "I think we both know X, can I ask her about you?"