Marriott dropped the ball at his feet to kick clear but failed to see Robbie behind him and the Irish star rolled the ball home.
Gus Poyet pulled back Christophe Dugarry for Paul Devlin's equalising penalty with 15 minutes left - but redeemed himself with a magnificent volleyed winner in the 87th minute.
City boss Bruce said: "Poor Andy's waited to play in the Premiership again, so for that to happen was very cruel.
"He put the ball down, as keepers do because they'd rather kick it off the floor than out of their hands.
"I saw Keane behind Andy but couldn't shout loud enough to tell him he was there.
"It was cunning by Keane and unfortunate for Andy because it will haunt him forever."
Bruce revealed his despair at the result but backed his men to stay above the bottom three as the relegation fight hots up.
"Sometimes football kicks you where it hurts and that was one of those games today," he said.
Steve Bruce suggested keeper Andy Marriott will be 'haunted forever' after Robbie Keane's opportunist goal against his Birmingham City side on Saturday.