Only results elsewhere have saved the Hornets from falling off their undeserved lofty perch until now, but they went into their clash with Bristol City again knowing they would still be top whatever happened after West Bromwich Albion failed to win at Crystal Palace.
It looked like Watford's advantage would revert back to two points after John-Joe O'Toole had come off the bench to cancel out Enoch Showunmi's second-half opener with his first senior goal, less than two weeks after breaking his international duck for the Republic of Ireland Under-21s.
But the confidence of Aidy Boothroyd's men is brittle at best after a run of only four points from 15 previously available, and in the third of four minutes of injury-time they failed to react when Mart Poom palmed away Michael McIndoe's cross, allowing the ball to be played back into the area for Darren Byfield to scramble home the winner less than two minutes after entering the fray.
That goal was greeted with jubilation by City boss Gary Johnson, who had seen his side slide down the table on the back of five games without a win. But while the three points were welcomed by the visitors, it probably slightly flattered them over the 90 minutes.
After a scoreless first half, the home side had the better of the second period until they were undone when McIndoe's cross from the left found three team-mates lining up unmarked at the far post and Showunmi won the race to nod past Poom.
However, Watford, who played with much more spirit than was on display in their dire midweek defeat at home to Burnley, deservedly got back on terms when Jay DeMerit flung himself full length to head a Lee Williamson corner back into the six-yard box and O'Toole glanced home.
A draw might have been enough to appease the increasingly restless Hornets faithful, but after the dramatic climax it was boos that once again rung out around Vicarage Road as Watford again looked nothing like the promotion certainties many had tipped them to be just a few weeks ago.