Substitute Albert Adomah scored the winner for Queens Park Rangers to end Watford's unbeaten start in the Championship.
Ilias Chair had given QPR a deserved lead with a fortuitous finish from 30 yards, which took a huge deflection off Francisco Sierralta on the way to finding the bottom corner.
Ken Sema restored parity with a calm finish after battling through Rangers' defence, only for Chris Willock to finish a fine team move to put QPR ahead once more.
Watford striker Joao Pedro hit the bar with a 40-yard effort and eventually equalised with a powerful low finish before Adomah sealed QPR's second win of the season.