The Championship leaders Sheffield United looked to be heading towards there second defeat of the season after substitute Dirus Henderson had put the home side 2-0 up before the Blades staged an incredible comeback.
Though it was Phil Jagielka and Paul Ifill who drew United level before former West Ham defender Malky Mackay was sent off for flooring Rob Kozluk in an off the ball incident. United then wrapped up the win with an Clarke Carlisle own goal.
The Hornets were the first team to dominate early on and created the first scoring opportunity, just four minutes in Matthew Spring broke free to knock the ball down to striker Ashley Young who slashed his shot over the crossbar.
The game turned sour for Watford, when just 17 minutes into the match top scorer Marlon King (6 goals) was rocked by an hamstring injury which forced boss Adrian Boothroyd to introduce former Gillingham striker Dirus Henderson to the field.
And, on 38 minutes the substitute should have put the home side ahead after making a error of Young's cross as full-back Rob Kozluk cleared for the Blades. Though the resulting corner did give desired effect, Young crossed home for the advancing DIRUS HENDERSON to put third placed Watford in front.
Despite being in front the injury to former Forest front man King dampened the Hornets performance as United slowly edged back into the game.
Midway through the half Paul Ifill struck to produce a great save from keeper Ben Foster while Nick Montgomery and Alan Quinn dominated in Midfield.
HALF-TIME: WATFORD 1-0 SHEFFIELD UNITED
Once again it was the home side who started the half brightly with Young's deflected shot tested Paddy Kenny before Henderson doubled the lead on 53rd minute.
David Unsworth and Leigh Bromby allowed Foster's long punt to bounce and DIRUS HENDERSON walked past Bromby to slide the ball past Kenny.
But, no sooner had the Hornets seen three points coming their way United sprung into action and saw two goals fly into the net within 10 minutes.
Winger Paul Ifill floated in a sensational cross for PHIL JAGIELKA to leap and head home a goal that would have been applauded in the Premiership, this was Jagielka's second goal in as many games.
The Blades then levelled midway through the second half, PAUL IFILL twisted and turned his marked to fire in a low 25 yard screamer which beat Foster at his near post.
However all was not over for Watford as the plot thickened, Kozluk fouled Henderson on the edge of the box and whilst Young was lining up a shot Mackay gave Kozluk some punishment. The former Norwich and West Ham man was sent off while Kozluk was cautioned.
United took full of their one extra man advantage and eventually won the game after a scramble in the box. Substitute Danny Webber crossed to Ifill is miss shot fell to Montgomery who shot at goal, this deflected off CLARKE CARLISLE and the former Blades target slammed the ball into the top of his own goal despite Montgomery claming.
Later on Carlisle was lucky not to join Mackay for an early bath when his reckless challenge, also on Kozluk received a yellow card from referee Mark Clattenburg when a red may have been produced on another day.
FULL-TIME: WATFORD 2-3 SHEFFIELD UNITED
Man of the Match: Phil Jagielka