Substitute Keith Treacy marked his Burnley debut by scoring a late equaliser to deny Sean Dyche a dream start to his Watford reign.
Rookie boss Dyche, 40, saw his side race into a 2-0 lead through Craig Forsyth and Mark Yeates.
But the Clarets recovered in the final 13 minutes as Charlie Austin pulled a goal back.
And fellow substitute Treacy - a summer signing from Preston - headed an 84th-minute equaliser.
Burnley, led by new skipper Chris McCann in the heart of midfield, made a bright and purposeful start.
They threatened in the fifth minute when midfielder Dean Marney hit a fierce right-foot shot which sailed just wide from 20 yards before Ross Wallace's effort drew a fine block from Adrian Mariappa.
The hosts were firmly in the ascendancy and moments later Jay Rodriguez engineered the space to hit a shot from six yards which missed the target.
But the pendulum swung in Watford's favour when Chris Iwelumo, a summer signing from Burnley, tried his luck from 18 yards with a shot that home debutant Ben Mee did well to block.
And Watford opened the scoring on the stroke of half-time when Marvin Sordell flighted a right wing cross for Forsyth to head into the bottom corner from six yards.
Burnley made a tactical switch at half-time as Martin Paterson was replaced by Austin in a bid to boost the home attack.
Yet Watford continued to defend strongly as Austin wasted a decent opening before Forsyth shot wide when well placed.
Yeates was denied by a smart stop from home keeper Lee Grant on 54 minutes and his endeavour was rewarded in the 70th minute when he drilled home a right-footed shot from 12 yards.
However, the Clarets responded in the 77th minute when Austin fired home from six yards and, with six minutes remaining, Treacy nodded in a cross from fellow wideman Wallace to level the scores.