Karl Darlow looked like being the hero on his return to the City Ground, keeping out two first-half penalties - both of which led to red cards for the visitors - but an unfortunate own goal for fellow returning player Jamaal Lascelles saw the Magpies fall just short of a miraculous point.
Victory for Forest takes their recent points tally to 10 from a possible 12, while United have now lost three games on the spin in all competitions to see their hopes of success falter.
Darlow was called into action for the first time 10 minutes in to deny Matty Cash, who managed to sneak in at the back post and force a good save out of the former Forest keeper.
The midfielder, already contesting an FA charge for alleged racial abuse, caught Henri Lansbury with his boot and was given the double punishment of a penalty behind awarded for his foul play.
More drama was to arrive before the break, though, as Darlow this time kept out Lansbury's penalty after Paul Dummett was shown a red card for a foul inside the box.
Down to nine men, United initially struggled to compete as Bendtner found a leveller following a goalmouth scramble, but the Toon Army restricted their opponents to minimum chances.
That all changed four minutes from time, however, thanks to a Lascelles own goal as the ball bounced off his midriff and over the line to cruelly deny Rafael Benitez's men a share of the spoils.