I've just seen a job opening at an Ivy League university, advertised as being open at either the Assistant Professor (tenure track) or Associate Professor (tenured) level. How common is this? Is it more likely that they have done this to widen the pool of potential excellent candidates, or because they already have someone in mind for the position, but need to advertise to satisfy an administrative requirement?
In my experience it is quite common for universities at all levels to advertise positions in this way so that they can hire someone at the associate professor level if there is a strong candidate with appropriate experience. I've been involved in many searches that were advertised in this way. In my experience most of these positions end up getting filled at the assistant professor level anyway.