A misnomer is a mistake, but not all mistakes are misnomers. The word is wrongly used in this sentence: It's a misnomer that elephants are afraid of mice. A misnomer is not the same as a misconception. The nome in the middle is from the Latin nomen, meaning "name." A misnomer is a mistake in labeling: for instance, calling aluminum foil "tinfoil" or calling a koala a "bear" (it's a marsupial).
If "Lucky" Brown loses his fortune in the stock market and "Speedy" Green blows out his ankle, their respective nicknames become misnomers.