Bankers Trust v. Mallis
In Bankers Trust v. Mallis, 435 U.S. 381, 386 (1978), the Supreme Court stated that judicial efficiency requires that the parties be able to waive the separate document requirement in order to avoid the pointless exercise of dismissing the appeal only to have it refiled when the district court enters the required separate document.
The Court then went on to find waiver in that case because the petitioner (who was the appellee at the circuit level) did not object to the taking of the appeal in the absence of a separate judgment. See id. at 387-88.