In Lovecraft's The Case of Charles Dexter Ward, the phrase "do not call up any that you can not put down" is oft repeated.
... doe not call up Any that you can not put downe; by the Which I meane, Any that can in Turne call up Somewhat against you, whereby your Powerfullest Devices may not be of use. Ask of the Lesser, lest the Greater shal not wish to Answer, and shal commande more than you.
Joseph Curwen was a very powerful necromancer; his spell against his descendant transcended time, and he even summoned Yog-Sothoth thrice. He was killed by a monster that he had raised but not managed to put down.
Dr. Willett, who knew nothing of necromancy and just happened to stumble upon the words of the unsummoning incantation, managed to put Curwen down. The latter wasn't even caught flat-footed.
Did I miss something, or was the doctor just very lucky?