Perhaps my favorite deliberate error in arithmetic is from Winston Groom's Forrest Gump, during the protagonist's tour in Vietnam:
> I am the machine gun ammo bearer, cause they figger I can carry a lot of shit on account of my size. Before we lef, a couple of other fellers axed if I would mind carryin some of their han grenades so’s they could carry more orations, an I agreed. It didn’t hurt me none. Also, Sergeant Kranz made me carry a ten-gallon water can that weighed about fifty pounds.
Unlike Forrest, the reader knows a gallon of water weighs roughly eight pounds and is not spared the extent of his burden.
Eighty pounds is "about fifty pounds", especially considering that a water can worth carrying at all is often not full, and insofar as nothing else in the paragraph is given a specific weight so the precise number doesn't really matter.
> the reader knows a gallon of water weighs roughly eight pounds
Haha, you have to love those quaint, backwards Americans and their “freedom units”. Meanwhile, in the rest of the world, we can peacefully enjoy the fact that 3.785 litres of water weighs exactly 3.785 kg.
> I am the machine gun ammo bearer, cause they figger I can carry a lot of shit on account of my size. Before we lef, a couple of other fellers axed if I would mind carryin some of their han grenades so’s they could carry more orations, an I agreed. It didn’t hurt me none. Also, Sergeant Kranz made me carry a ten-gallon water can that weighed about fifty pounds.
Unlike Forrest, the reader knows a gallon of water weighs roughly eight pounds and is not spared the extent of his burden.