"Would you like some butter on your bed?" Take the margarine off the quilt! You've simply stumbled across a parapraxis.
From the Latin para meaning "beside" and the Greek praxis, for "a doing," a parapraxis is an instance in which you say one thing and mean your mother . . . er, another. The term is most commonly known as a Freudian slip and was deeply instrumental in the work of psychoanalyst Sigmund Freud in determining his patients hidden intentions and desires.