The number 13, commonly associated with misfortune, appears as a common (albeit subliminal) motif throughout the fight against Mr. Sandman:
• His full nickname, Mister Sandman, has 13 letters.
• He is the 13th opponent in both Contender and Title Defense modes; reciprocally, Little Mac is Mr. Sandman's 13th opponent, who Sandman loses to.
• Little Mac has 13 hearts by default in both fights.
• Mr. Sandman's introduction in Contender mode has 13 frames, as opposed to the usual 4.
• When Mr. Sandman gets tired after the dreaded Berserker Rage in Title Defense, Little Mac only needs to punch him 13 more times to defeat him (although obviously less punches are required if stars are used).
Contributed by game4brains