Nottingham Forest 2 Derby County 3
Forest had looked to be cruising after powering their way to a 2-0 lead within the first 14 minutes, but found themselves dumped out of the competition by their fiercest rivals following a classic Cup encounter.
Rob Hulse began the fightback before half-time with a firm header and Paul Green pulled the Rams level on the hour mark - before Commons put the visitors through with a dramatic winner against his former side.
Forest could not have made a better start to the game as, following a short corner routine, Chris Cohen was given the room to stride across the face of the box and strike a powerful, rising shot that bulged into the back of the net.
It was enough to stoke the already intense atmosphere into a maelstrom of noise, as both sets of fans roared their sides onwards.
But it was the home supporters who had more to cheer about as Forest continued to carve out further chances, with Nathan Tyson agonisingly close to latching onto a Lewis McGugan throughball and Matt Thornhill flicking a header wide.
When Cohen slipped the lively Tyson through the offside trap, it was the catalyst for a second Forest goal.
The striker's pace saw him win the race for the ball with keeper Stephen Bywater, whose desperate challenge was only enough to haul the Forest man down.
The Derby stopper escaped severe punishment as referee Chris Foy decided the offence was only worthy of a yellow card - but he could do little as Tyson regained his feet to lash the resulting penalty high into the top right-hand corner.
Commons, who was receiving a barrage of abuse from the fans of the club he left to join Derby, almost silenced his critics with a fierce shot that drew a smart save from Paul Smith.
But, after a short spell of Rams pressure, the keeper could do little as Hulse rose to head a goal back for the visitors, planting a firm downward effort low into the net as he connected with Gary Teale's cross.
Cohen forced a falling save from Bywater with a rasping effort from the edge of the box - but it was Derby that were to find the back of the net for a second time.
Again it was Teale who delivered into the danger zone, with Green this time applying the headed finish.
And the visitors' comeback was completed in predictable fashion, as a familiar face returned to haunt his former club when Commons put Derby ahead for the first time.
The forward had already issued a warning with a 25-yard shot driven against the foot of the post.
But, when he tried his luck again from an identical position, a helpful deflection off defender Luke Chambers helped guide the ball inside the post.
And, while Garath McCleary, Wes Morgan and Mark Byrne all had chances to equalise, Derby held on for the victory.