Former Watford striker Danny Webber scored a controversial winner against his old club to give Sheffield United a crucial win in their relegation fight.
Sheffield United defended resolutely at Vicarage Road
Substitute Webber headed in from close range in the 88th minute, though replays suggested he was offside.
United probably deserved the win given both Christian Nade and Rob Hulse had struck the post in the second half.
Chris Powell was red carded in stoppage time after picking up a second yellow card to compound Watford's misery.
If United's win was dramatic in its timing, the preceding 87 minutes provided plenty of evidence both these sides will do well to be playing Premiership football next season.
The ball was in the air as much as it was on the floor, with neither side seemingly capable or even willing to string a series of passes together.
It made for grim watching.
The desperate search for Premiership points was such that the Vicarage Road ballboys would only dry the ball with a towel for the home team when it went out of play for a throw.
That lack of sportsmanship had Neil Warnock angrily prowling the touchline and he became so animated at one point he sarcastically proferred his own towel, throwing it down on the side of the pitch.
Early on Tommy Smith briefly illuminated proceedings with a run and a shot that went narrowly over the bar.
Darius Henderson's instinctive volley then drew a reaction save from Paddy Kenny, with Chris Morgan clearing the ball for a corner.
Another Watford shot, this time from Gavin Mahon, caused more problems for the visitors in that Claude Davis was injured in trying to block the ball and had to be replaced by Leigh Bromby.
United had few chances in that opening half, but Warnock's side probably had the best openings when after a bout of aerial ping-pong the ball dropped into the path of Nade.
Making his first Premiership start, Nade only had Watford goalkeeper Richard Lee to beat but the ball did not bounce kindly for the Frenchman and the danger passed.
In the first minute of the second half Nade outsprinted Malky Mackay but embarrassingly miscued in trying to pull the ball back to Keith Gillespie.
On the other flank Derek Geary delivered two good crosses that troubled the Watford defence.
Hulse was just unable to reach the first cross, but when the second came over soon afterwards Nade crashed a header against the upright.
Just past the hour Nade missed a third chance when he blazed over the bar following a cross from Gillespie.
United continued to create the better chances and Hulse struck the woodwork for a second time when it looked easier to score.
Watford manager Aidy Boothroyd's solution to United's second-half dominance was to introduce giant centre-back Dan Shittu and deploy him as a forward.
But it was United's substitute Webber that changed the course of the game when he was on hand to head in following a goalmouth scramble.
Watford might have had a penalty when they claimed Rob Kozluk had handled and soon after Powell was dismissed for collecting a second yellow card.
Watford manager Adrian Boothroyd:
"Danny Webber's goal was offside but let's be right about things, we didn't perform.
"They hit the post twice and deserved to win, simple as that. We didn't play our way, didn't compete, and things we are normally very good at we didn't do.
""If we lie down now then we've got a problem but I'm sure we'll bounce back."
Sheffield United manager Neil Warnock:
"We were determined to win and I thought we showed that.
"It was the biggest game of the season - we had said before if you want to stay in the Premiership you have to win, make no mistake about that.
"It's absolutely fantastic to come away with three points. It was hard work but I thought we were magnificent."
Watford: Lee, Chambers, Mackay, DeMerit, Powell, Spring (Doyley 46), Francis, Mahon, McNamee (Shittu 72), Smith, Henderson.
Subs Not Used: Chamberlain, Stewart, Priskin.
Sent Off: Powell (90).
Booked: Mahon, Henderson, Powell.
Sheff Utd: Kenny, Kozluk, Morgan, Davis (Bromby 30), Geary, Gillespie, Leigertwood, Jagielka, Alan Quinn (Kazim-Richards 83), Hulse, Nade (Webber 73).
Subs Not Used: Montgomery, Law.
Goals: Webber 88.
Ref: M Atkinson (W Yorkshire).