The case
Picture the little man on the Chance cards, the one who dashes across the board with bags of money. Most people add a monocle. Look at any edition of the game and he doesn't have one. He never has.
The monocle comes from another top-hatted mascot of the same era: Mr. Peanut, who has worn one since 1916. Two dapper cartoon gentlemen in top hats, one accessory, and memory splits the difference.
Why our brains do this
Memory stores a category, "old-timey rich guy," and reconstructs the details from the category's props. Monocles are a prop of that category.
Sources: Wikipedia — Rich Uncle Pennybags · Wikipedia — Mr. Peanut · Wikipedia — False memory: the Mandela effect
