Swansea defender Alan Tate put through his own net in the 88th minute to hand Leyton Orient a 2-1 win and send them into the FA Cup fifth round for the first time in 29 years.
Jimmy Smith headed Russell Slade's men ahead in the 35th minute, but the Swans levelled on the stroke of half-time through Cedric Van der Gun.
Yet the real drama was still to come as Tate diverted a cross from substitute Paul-Jose M'Poku into his own net to send the League One club into the next round.
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