Shefki Kuqi converted an injury-time penalty to give Oldham a dramatic victory at nine-man Sheffield United.
Matthew Lowton scored from a Ched Evans pull-back before Evans made it 2-0 to the Blades with a low 18-yard shot.
But Richard Cresswell headed a Filipe Morais corner into his own net and Lowton was dismissed for two bookings.
Kieran Lee's 15-yard effort made it 2-2 before Harry Maguire was sent off for a professional foul on Reuben Reid and Kuqi scored from the spot to win it.
Promotion-chasing United, who had won their eight previous home league matches, missed the chance to move seven points clear of the play-offs and are now four points ahead of third placed Sheffield Wednesday.
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Sheffield United manager Danny Wilson tells BBC Radio Sheffield:
"Obviously massively disappointed in the manner that (the defeat) happened. But I think overall we can only blame ourselves.
"We're not looking at any scapegoat from certain people in the middle in black, which we possibly could do, we have an have an argument in one or two cases, but we have to look at ourselves.
"We got to 2-0 up, played some decent football opened them up on numerous occasions and (couldn't) finish the game off.
"So we've got to look at ourselves and we've got to do better."