Paul Hart, the forest manager must have been cursing in the dugout and regretting the day he let Jack Lester go when the former Grimsby Town man put the Blades in front.
Forest missed chances either side of Lester's 33rd minute penalty and they were left to regret there sloppiness in front of goal.
Midfielder Andy Reid was clearly there match winner as he created chances, he rifled in a 18 yard left foot shot before setting up Louis-Jean who flashed his effort wide of the post.
Marlon King was then inches away from toe-poking home a low Reid cross.
But Louis-Jean brought down Michael Tonge in the 33rd minute, Lester rifled home the spot kick into the top left-hand corner.
Forest should have been on level terms three minutes before the break, with King setting up Gareth Taylor, but the ball sliced off the striker's right boot.
Lester had a chance to kill off the game soon after the restart but nudged a soft four-yard shot into the grateful arms of keeper Darren Ward.
Forest again took control following that chance but continued to waste chances.
It took a superb full-stretch save from Kenny in the 50th minute to keep out Gareth Williams' crisp, low, 20-yard drive.
Danny Sonner then fired into the side-netting soon after with the goal gaping, before Kenny again kept his side's nose in front by superbly pawing away a rising Sonner drive bound for the top corner.
From the ensuing corner the Forest fans were again left shaking their heads after Taylor contrived to head narrowly over after being left unmarked from six yards.
A dejected-looking Forest were then hit late on, with Morgan striking a spectacular scissor-kick after captain Rob Page had nodded on a Tonge corner in the 79th minute.
Tonge was again the supplier in the 90th minute with an inch-perfect cross for Allison to head home the final nail in Forest's coffin.
Man of the Match: Chris Morgan