A malapropism is what happens when someone attempts to use a word, but instead uses a similar-sounding word with a different meaning. It's the quintessential "wrong place, wrong time" of language. Accidental malapropisms in speech are simply caused by the brain reaching into the filing cabinet next to the one it wanted. They are quite common, and although they can be embarrassing (e.g. Freudian slips), they're usually laughed off without incident. They're a little more serious in formal speeches, such as those given by politicians, where the speaker is expected to sound intelligent.