The University of Texas reviews the performance of tenured profs and sometimes finds that they are getting paid for doing just about nothing, as in the lead case mentioned in this article. Even when problems continue for years (not showing up for class, not publishing anything), university officials “work with” the professor on these issues. My guess is that they will be working with her on them until she retires.
Apparently, these reviews rarely turn up instances of abuse of tenure, but is that because so few profs are inclined to do that, or because the people doing the reviewing tend to overlook all but egregious misconduct?