This pair of words is often mispronounced and misspelled. Casual, meaning "relaxed," "unconcerned," "not planned," is pronounced "KAZH'oo-uhl" or "KAZH-yoo-uhl"; causal means "involving a cause" or "expressing a cause" and is pronounced "KOzel," with the o sounded like aw in paw. The difference in meaning between these words is illustrated in this sentence: "The causal element in his failure was his casual approach to work."
Dictionary of problem words and expressions. Harry Shaw. 1975.