Watford's automatic promotion dream turned into a nightmare after losing goalkeeper Jonathan Bond to a horror injury.
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Bond was taken to hospital after a sickening collision while the Hornets could not take advantage of Hull's late slip-up against Cardiff.
The Bluebirds' last-gasp penalty equaliser against second-placed Hull raised the roof at Vicarage Road, where there were still some 10 minutes to play following the lengthy treatment required by Bond.
But Watford, who had also lost striker Troy Deeney to a red card, could not find the goal which would have taken them up.
Instead, Ross McCormack broke away down the other end and grabbed a late winner for Leeds.
Bond was only on the pitch after regular keeper Manuel Almunia injured himself during the warm-up.
But midway through the first half the 19-year-old bravely came out to meet Dominic Poleon as he raced Ikechi Anya for the ball, and the Leeds striker appeared to nudge Anya into the onrushing goalkeeper.
Poleon was booked for the challenge while Bond was carried off on a stretcher in a neck brace and oxygen mask and taken for X-Rays on a nasty facial injury.
Watford's rookie third-choice keeper Jack Bonham, who was not even in the squad half an hour before kick-off, came on for his professional debut.
To add insult to injury it was Poleon, himself on as a sub for the injured Steve Morison, who put Leeds ahead.
In the 42nd minute Poleon again chased a long ball over the top into the area.
This time the striker checked his run, Joel Ekstrand's clearance hit him in the chest and as defender and goalkeeper collided, Poleon nipped in behind to tap the loose ball into an empty net.
But the Hornets hit back during no less than 16 minutes of time added on at the end of the first half.
Anya made a surging run across the penalty area and played the ball to Almen Abdi, who curled in a superb first-time shot from the edge of the area for his 12th goal of the season.
Things got worse for Gianfranco Zola's side when they lost Deeney to a second yellow card in the 60th minute for a foul on Michael Brown.
However, the 10 men battled on and midway through the second half substitute Matej Vydra brought a fine diving save from Paddy Kenny.
Then came news of the dramatic events at the KC Stadium and Watford, who now knew a win would secure promption, suddenly had a spring in their step.
Kenny saved from Nathaniel Chalobah and Fernando Forestieri as they piled forward looking for the winner.
But instead a minute from time McCormack was put clear at the other end and chipped the stranded Bonham who could only fingertip the ball up and over the line to condemn Watford to the play-offs.