For instance, the expression to call a spade a spade, has absolutely nothing to do with race. People think that because spade is a rude term for a person with brown skin, the expression is derogatory toward brown people. However, the expression predates the racist epithet by about a zillion years. The first use of the expression in English dates to a 1542 translation of Plutarch:
Phillippus aunswered, that the Macedonians wer feloes of no fyne witte in their terms but altogether grosse clubbyshe and rusticall, as they whiche had not the witte to call a spade by any other name than spade.*Similarly, there's nothing offensive about the expression word of thumb. The myth goes that there used to be a law that a man could hit his woman as long as he did it with something less than the width of his thumb. Actually, it is possible that some English judge in the 18th century did release some sort of proclamation that men could beat their wives with things narrower than their thumbs (although that law, if it existed, wasn't ever called the rule of thumb). However, the expression predates the law by about a century in the English language alone, and was used in other languages even before that. No, the expression just refers to estimating the size of something using your thumb to represent an inch.
Fun fact: In my house, we measure things in Brigids.
There's also the rumor that the kids' rhyme Eeny meeny miny moe has racist origins, in that originally, the line catch a tiger by its toe was catch an n-word by its toe. Now it's true that this variation was used as far back as the 19th century, but variations on the rhyme predate that. That did not stop two women from suing Southwest Airlines after a flight attendant urged customers via intercom to find their seats by saying "Eeny meeny, miny, moe, pick a seat we gotta go." Two brown skinned women were sure that this was directed at them, and one was so upset she had some seizures. Really guys? Really? I think I err on the side of knee-jerk liberal most of the time, but wow dude. Wow.
*The only offensive thing about this passage is the spelling. That's because there wasn't really any such thing as standardized spelling back in the day. Shakespeare himself never quite decided how to spell his name (although if you look at examples of his signature, it's hard to imagine how you can even tell what letters he was trying to write). So the next time some snooty snob gives you crap about the destruction of the English language, you tell them "If spelling your name wrong is good enough for Willy S. it's good enough for Bridgitte Duall Brookway."
Or in the words of Fifty Shades (okay, this is the real reason I loved the books), "Prescriptive linguistics are a HARD LIMIT."