Can the USPTO ever use references which postdate applicant's effective date of filing?

Yes, in certain circumstances.  Such references can be used to show a universal fact--one that, for example, indicates that applicant's invention could not be built without undue experimentation, that was inoperative as described, or that it lacked utility.  Such references can also be used to help ascertain the skill level of a person of ordinary skill in the art at the time applicant's patent application was filed.  See ​MPEP 2124.