I recently did SFO-SMF-SFO-SMF-SFO one afternoon one a star RTW to get to 50k to requal for Prem Exec on United.
None of the UA employees really cared too much, although leaving SFO they refused to issue me boarding passes for the entire trip after consulting a supervisor. Apparently they felt there was some kind of security issue if they did issue me all the passes at the start - not sure what.