G-Fafif wrote: Consider the entire 2002 regular-season oeuvre of young and phenomenal Francisco Rodriguez. It was comprised of five games, the first of them appeared in on September 18. He spent the bulk of October shutting down waves of MFYs, Twins and Giants. Nobody checked his August 31 ID. MLB later admitted that they should NOT have approved K-Rod's admittance onto the post-season roster (they apparently misinterpreted their own guidelines) but by the time they did so it was after the Angels (one and only) WS was in the bag. They've since tightened the rules somewhat from those days anyway. Steve Phillips ('amongst others) exploited a loophole which allowed them to stick some single-A player on the 40-man, then DL him, then replace that "injury" with a newly added in September player. He pulled it off at least once in either the '99 or '00 teams (maybe both?) and others were doing to same. But, in general, yeah, being on the actual active roster on Aug 31st is certainly not the sole requirement.