Give a listen:
That first bit, a partody of "Molly Malone" (AKA "Cockles and Mussels" or "In Dublin's Fair City"), contains the line "My Molly stands out 'cos she weighs eighteen stone" followed by the parenthetical comment, "That's 256 pounds." Problem is, it's not. A stone is (or, more precisely, was when the song was written) 14 pounds. So 18 stone is equal to 252 pounds.
I am guessing that Sherman's mistake came about by assuming that (x+a) × (x-a) = x × x and therefore that 18 × 14 = 16 × 16 = 256. That's an error that's common among the inumerate. I remember when I was in college, working on the campus newspaper. A friend enlarged a photo to 125% of its original size. Then shrank the enlarged copy to 75% of its size. He was puzzled that the final version was smaller than the original.
I know it's not really a big deal. But it still hurts my ears every time I hear that line.
As a consolation prize, let me share with you a video I found while looking for the one posted above: