Former Forest winger Commons sent the Rams through to a last-16 tie at home to Manchester United later this month with a deflected second-half effort.
Derby had seemed down and out when they quickly fell two goals behind. Chris Cohen gave the Reds a dream start with a goal after only 60 seconds.
The midfielder collected a return pass from his own short corner and, after taking a touch to make the shooting angle more favourable, curled a left-foot shot past the unsighted Stephen Bywater from the edge of the area.
Nathan Tyson doubled their lead from the penalty spot in the 14th minute, after he sprang the offside trap and raced goalwards before being tripped by Bywater.
Bywater was shown a yellow card and was arguably fortunate not to see red after denying Tyson a clear-cut scoring opportunity as the last man.
The Rams halved their deficit in the 28th minute when Rob Hulse rose unchallenged at the far post to guide Gary Teale's left-wing cross into the opposite corner.
But Derby were in the ascendancy and they equalised on the hour when Paul Green took advantage of some more statuesque Forest defending to head home another Teale cross.
Green dribbled a shot narrowly wide midway through the second half before a thunderous shot from Commons - reminiscent of the one he unleashed in the Carling Cup semi-final first-leg defeat of Manchester United - cannoned back off the foot of the post.
Commons finally found the net in the 73rd minute to score the decisive goal. The Scotland international wrong-footed Wes Morgan and created a yard of space for himself.
His shot clipped Morgan as the Forest defender tried to recover and the ball looped beyond Smith and into the far corner.
Substitute Mark Byrne had a golden opportunity to force extra-time with two minutes remaining but the youngster took a heavy touch when trying to control Morgan's cross and scuffed his far-post shot wide as Bywater closed in on him.
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