The defender got on the end of Jesus Navas' cross right on the stroke of half time, attempting to clear it acrobatically.
However, he unwittingly guided the ball into his own net beyond a stunned Thibaut Courtois, with City players wheeling away emphatically not believing their luck.
And Twitter could likewise only celebrate what will be remembered as a special goal by Cahill, though not quite the way he intended.
Of all things, a @ManCity goal from a Navas cross. No, of course he didn't find one of his teammates. Cahill own goal.— Gary Lineker (@GaryLineker) December 3, 2016
Gary Cahill has just scored the goal of the season for Man City. WHAT A GOAL!!— Troll Football (@TrollFootball) December 3, 2016
OHHHH YEESSSSS WELCOME TO THE PUSKAS AWARD GARY CAHILL pic.twitter.com/hMExP8HUOM— Trey (@Vintageilkay) December 3, 2016
Despite the best attempts of Anthony Taylor, City take the lead!— City Watch (@City_Watch) December 3, 2016
Navas' cross is met at the near post by Gary Cahill who finishes superbly.
Top class defensive entertainment from the Englishman, who capably usurped team-mate David Luiz in the role of providing comedic relief on Saturday...