4th down, field goal attempt by Baylor. Blocked by SMU, who clearly gains possession but fumbles. Baylor lineman recovers and falls down but "throws" pass forward. Flagged for illegal forward pass. Replay (correctly) shows Baylor player down before pass is thrown, thus no foul. Baylor gets possession since SMU possessed ball and lost it.
However, had there been an actual illegal pass thrown before the Baylor player was down, how do you enforce the loss of down penalty? Would it be second down? Or would the loss of down conflict with other rules -- most notably that the down is irrelevant because of legal possession by Team A for the second time in the down?