Interview for a faculty position after accepting a postdoc offer
My understanding of the norms in academia is that tenure-track and tenured positions are generally accepted to be rare opportunities, and that taking those if offered is the right thing to do. As such, it is perfectly acceptable to attend an interview for a tenure-track position, and to accept it if offered, even if one has already accepted a conflicting postdoc. In such case one should, however, be apologetic to the snubbed postdoc mentor, and do ones best to minimize the inconvenience to them.