Forest went ahead through Nicolas Dominguez before Marcus Rashford pegged them back only for Morgan Gibbs-White's strike earning them all three points on Saturday night.
The first goal came from Dominguez, who was completely free just inside the box to tuck home countryman Gonzalo Montiel's cross after an hour on the clock.
United had not even had a shot inside the box prior to the goal while just moments before, Rashford was on the deck.
While the England international has been able to continue, it proved to be the right decision as he went on to grab the equaliser.
Forest goalkeeper Matt Turner tried to pass out from the back but it was easily intercepted by Alejandro Garnacho.
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And the winger slid the ball across to Rashford, who planted a perfectly placed shot into the corner.
But the match swung back on its head when Turner pulled off a fine save against Christian Eriksen that sparked a counter-attack.
Led by the pacy Anthony Elanga, once of the Red Devils, his pulsating run found Morgan Gibbs-White on the edge of the box.
Much like Dominguez, he had been left completely on his own and had time to place his shot.
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And the former Wolves star did exactly that, kissing the inside of the net with Andre Onana sprawling.
It made sure Erik ten Hag ended 2023 with more questions than answers with the pressure growing by the week.
With Sir David Brailsford, INEOS' director of sport, in attendance next to Sir Alex Ferguson, the United boss might be having some nervous thoughts.
With tonight's defeat, United are as close to Forest in the Premier League than they are the top of the table with 11 points separating them.
Only two teams in the league have scored less goals than them - Burnley and Sheffield United.