Sooner or later, someone like Russell Bucklew was going to come along and throw a big wrench into the predictable back-and-forth debate over the constitutionality of executing people by lethal injection.

On Wednesday night, just hours before Mr. Bucklew was scheduled to die in Missouri, the Supreme Court granted him a rare stay of execution after medical professionals found that an unusual congenital disorder would likely cause him to suffer on the executioner’s table.

Mr. Bucklew, 46, was sentenced to death for killing his ex-girlfriend’s boyfriend in 1996, and then abducting, beating and raping her. He challenged the state’s plan to put him to death by lethal injection on the grounds that a condition called cavernous hemangioma — which has led to expanding vascular tumors in his head and neck — would expose him to “unique risks,” including “a substantial likelihood of hemorrhaging, choking, airway obstruction and suffocation.” The justices sent the case back to the lower courts to decide whether to hold further hearings.

Lethal injection has already come under increased scrutiny following multiple botched executions, most recently Oklahoma’s appalling 43-minute torture of Clayton Lockett last month. Multiple legal challenges to the procedure have centered on whether states may keep secret the drug protocols they use and the shady compounding pharmacies that make them.