In any castle level (x-4 worlds), touch the axe when the timer reaches zero. (You may want to fireball Bowser out of the way first.) Mario will go save the Mushroom Retainer as usual, but then the timer will shoot up to 999 and start counting down to 0, giving you 50,000 points!

This bug will not work in the original NES version of Super Mario Bros. because you do not get the timer bonus in the castle levels.