Metathesis [meh-tath-uh-sis] refers to the transposition of two sounds or letters in a word, like a vowel and a consonant, or of words in a sentence. For example, "calvary" is a metathesis of "cavalry."
Captain Jean-Luc Picard referred to Lieutenant Reginald Barclay as "Mr. Broccoli," an apparently inadvertent use of a derisive nickname ascribed to Barclay by other members of the crew. Lieutenant Commander Data attempted to excuse Picard's slip of the tongue by pointing out that "metathesis is one of the most common of pronunciation errors." (TNG: "Hollow Pursuits")