Portsmouth manager Steve Cotterill must have thought his side had pulled off one of the results of the day thanks to Kanu's goal and even when Ibrahima Sonko put through his own net with three minutes to go would have been happy with a point.
Forest had other ideas however and Marcus Tudgay sparked frenzied scenes of celebration when he scored in the fourth minute of injury time.
Over 21,000 watched the Reds keep the pressure on in their chase for a play-off place with Billy Davies's side now just two points adrift of the top-six and only six points behind the automatic promotion places.
Reaction to follow on Forest Mad.