The occassion in which the undergarments used to support the breasts are too tight and cause the breasts to spill out from the edges. This could occur in the cleavage, under the arms, and/or on the back.
As heard in the Hamptons: "Pinky's cups certainly do runneth over tonight, darling... Pass me another watercress sandwich."
As heard in South Central LA: "Damn, girl, you need not be wearin' that halter top when you got all that boob-ooze goin' on!"