Substitute Vivaldo Semedo's equaliser in the seventh minute of stoppage time denied Sheffield Wednesday a second Championship win of the season.
Charlie McNeill's third goal in two games appeared to have handed rock-bottom Wednesday a rare bit of cheer in a difficult campaign.
But just when wasteful Watford, who missed a penalty through skipper Imran Louza, seemed to have run out of ideas, Semedo squeezed a volley from Mamadou Doumbia's cross under goalkeeper Pierce Charles to salvage a point.
Struggling Wednesday were relieved not to fall behind inside five minutes when Giorgi Chakvetadze strike was too close to goalkeeper Charles from Othmane Maamma's low centre.
But from their first meaningful excursion into Watford territory, the Owls went ahead after 17 minutes.
Max Alleyne's poor headed clearance from Yan Valery's cross fell straight to McNeill and crisp left-foot shot extended the Hornets' dismal sequence of just one clean sheet since April.
It was also the eighth time in 11 home games this season that Watford have conceded the first goal.
And the division's comeback kings, who have rescued 19 points from losing positions this season, left it desperately late to add to their tally.
Louza placed his shot just the wrong side of a post from fellow Moroccan Maamma's pass, before the Golden Ball winner at the Under-20 World Cup was guilty of a glaring miss from six yards.
With loyal veteran Barry Bannan marshalling the Owls' resistance, Henrik Pedersen's men somehow held out until the break.
As Watford laid siege to the visitors' goal, hapless referee Dean Whitestone's inadvertent body-check on Marc Bola almost set up a Wednesday breakaway, and the visitors were upset when the experienced official stopped play for a drop-ball.
Whitestone was the centre of attention again when Louza's penalty, awarded for a foul on Bola by substitute Sean Fusire, rattled the crossbar and Edo Kayembe steered home the rebound.
But Kayembe was penalised for encroaching and Watford's night of missed opportunities reached a frustrating new low.
Louza did his best to atone for his spot-kick miss, twice extending Charles from long range, and Wednesday looked like holding out until Semedo's second goal of the season broke the 1,578 travelling fans' hearts.
The managers
Watford's Javi Gracia:
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Sheffield Wednesday's Henrik Pedersen:
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