Newcastle left it late, but Bruno Guimarães' 94th-minute diving header gave them a fifth successive home victory on Sunday.
The Brazil international had already equalised for United in the first half after Ademola Lookman had given Leicester the lead, but the game was heading for a draw when Matt Targett played the ball down the line to substitute Joe Willock in the dying seconds.
He left Youri Tielemans trailing in his wake and advanced into the box before his cross took a deflection and looped into the air, with Guimarães arriving from nowhere to head home and spark bedlam amongst another sold-out crowd at St. James' Park.
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