The adjective nuclear often comes up in discussions of common mispronunciations thanks to a handful of very famous and well-educated people who have demonstrated difficulty producing this particular series of letters. Rather than saying [noo-klee-er], some speakers say [noo-kyuh-ler], which is sometimes spelled out as nucular. In linguistic terms, this is an example of metathesis, or the transposition of letters, syllables, or sounds in a word. In this case, the l sound is moved from the second syllable to the final syllable. George W. Bush famously mispronounced nuclear, but he wasn’t the first US president to do so; Bill Clinton, Jimmy Carter, and Dwight D. Eisenhower also succumbed to the perils of this specific example of metathesis.