Sheffield United striker Jack Lester scored a penalty to help dump former side Forest out of the FA Cup.
Lester slotted home in the first half after Matthieu Louis-Jean was adjudged to have fouled Michael Tonge.
Forest dominated for long spells, Andy Reid, Marlon King and Gareth Taylor all wasting goalscoring opportunities.
Paul Hart's side piled forward after the break but Taylor headed over, before Chris Morgan's volley and Wayne Allison's neat finish ended the tie.
The two teams have become fierce rivals in recent times, with last May's dramatic play-off semi-final when Neil Warnock's side won after extra-time still fresh in the memory.
And there was no love lost on the pitch in the early stages as Stuart McCall suffered a nasty looking shin injury, though the veteran managed to carry on.
Forest had the first half-chance, Eugen Bopp's corner not properly cleared by the Blades and King seeing his acrobatic overhead-kick fly over the crossbar.
23 mins: Louis-Jean heads wide
34 mins: Lester converts from the penalty spot
62 mins: Taylor heads over from three yards
79 mins: Morgan volleys in
90 mins: Allison scores
Reid had a good opening himself moments later, but his sweetly-struck left-foot volley curled straight into the arms of Paddy Kenny.
Forest were beginning to dominate, and Louis-Jean headed wide from a Reid free-kick when he should have hit the target.
But Hart's side were hit with a sucker-punch when Tonge fell under a Louis-Jean tackle and referee Phil Dowd pointed to the spot, Lester slotting into the roof of the net.
Forest were incensed by the decision and attacked with renewed vigour, Bopp having a shot cleared off the line before Taylor lazily slotted wide after good work from King.
Forest had scored just once in their previous six games, and no matter how much possession they had they just could not put the ball in the net.
Bopp saw a shot deflected wide just after the break and then Gareth Williams' low drive was well-saved by Kenny.
As Forest continued to push for an equaliser two glorious chances went begging.
First Sonner sliced horribly wide from the right and then Taylor headed over from three yards when it looked easier to score.
And the Reds were hit by another counter-attack as United killed the tie off with 11 minutes left, Morgan volleying home from Tonge's cross.
Wayne Allison rubbed salt into the Forest wounds in the last minute as he converted a cross from close-range.
It was just United's third shot of the game, and it was rough justice on a Forest side that now must seek to maintain its Division One status.
Nottm Forest: Ward, Louis-Jean, Walker, Doig, Robertson,
Sonner, Williams, Bopp (Westcarr 80), Reid, King, Taylor.
Subs Not Used: Jess, Roche, Gunnarsson, Gardner.
Sheff Utd: Kenny, Kozluk, Page, Morgan, Wright, Montgomery,
McCall (Rankine 90), Tonge, Lester (Whitlow 75), Allison,
Peschisolido (Forte 60).
Subs Not Used: Sturridge, Armstrong.
Booked: Kozluk, McCall.
Referee: P Dowd (Staffordshire).