Team A scores a touchdown (6 points) and attempts to kick an extra point. It is blocked. Under the old rules, the play is over; under the new rules, either team can pick it up and run. Team B does so, picking it up and running downfield. Before reaching the goal, the Team B player fumbles the ball and a chasing Team A player picks it up. The Team A player is surrounded by Team B players and attempts to evade them, but only succeeds in accidentally backtracking into his own end zone, where he is tackled.
The result of the play is a safety, which is usually worth two points - but since this is a two-point conversion attempt (after the kick is blocked), the safety is only worth one. So the score is 6-1 in favor of Team A.
The odds of such a crazy play are infinitesimal. One-point safeties have happened a few times in college, where it's been legal for decades, but have always been scored by the offense. I am eagerly awaiting the first one-point defensive safety.