The Latics appeared to have the points in the bag thanks to Richie Wellens' second minute strike, a goal from Paul Warne one minute before the break and Chris Porter's 19th of the season early in the second half.
Northampton - who were to suffer their first defeat under new manager Stuart Gray and their first loss in six matches - played their part in an entertaining spectacle and were on the scoresheet through Andy Kirk and debut loan signing Jordan Robertson.
Forest lost 3-0 to Chelsea at Stamford Bridge in what will go down as one of the most one-sided games in the history of the competition.
Goals from Andriy Shevchenko, Didier Drogba and John Obi Mikel chopped down Forest at Stamford Bridge in a match where the visitors hardly had an attack on target.
Chelsea dominated from the start and Shevchenko forced a fine one-handed save from goalkeeper Paul Smith within three minutes of the start when his right-footed piledriver from the corner of the six-yard box was parried to safety.
Chelsea went ahead on nine minutes when Shevchenko latched on to a weak clearance by James Perch to rifle the ball towards goal from six yards. The shot deflected off John Curtis and sent Smith the wrong way as the ball hit the back of the net.
Goal number two was inevitable and it came from the boot of Drogba on 18 minutes.
Nicky Southall had hauled down Wayne Bridge 25 yards from the Forest goal. Frank Lampard appeared to run up and shoot but jumped over the ball, leaving the Ivorian striker to curl a powerful right-footed free-kick high over the defensive wall and beyond the grasp of Smith.
Shaun Wright-Phillips almost made it three on 26 minutes but his snatched shot from 12 yards snaked past the Forest upright.
On the stroke of half-time and following a Lampard corner which wasn't cleared, Mikel toe-poked the ball home right on the line for number three.
The second half was much as the first with Chelsea in complete control.
Despite this Junior Agogo broke free into the Chelsea penalty area on 58 minutes and sent in a low drive across the body of Petr Cech. Unfortunately for the League One side, nobody was able to get a final touch to the ball in their only attack of the game.
Salomon Kalou replaced Drogba on 61 minutes and within four minutes he almost scored with a powerful header from ten yards but Smith did well to claw it away.
Smith again was in the thick of it this time hacking away a Lampard trademark rasper on 68 minutes.
Chelsea could have had two goals on 75 minutes. First Curtis kicked a powerful header by Michael Essien off the line and then Smith was quick to collect a snap-shot by Lampard.