Queens Park Rangers earned their first victory under boss Gareth Ainsworth and ended a 13-game winless run by condemning Watford to defeat in Chris Wilder's first match in charge.
Tim Iroegbunam did well to slide the ball into the far corner for QPR's decisive goal, with the 19-year-old's winner an early setback in Wilder's tenure after he took over from Slaven Bilic just four days earlier.
Watford struggled to create chances to equalise, with Imran Louza wasting their best opportunity when he headed wide from a Joao Pedro cross with 20 minutes remaining.
In a frenzied finish, Pedro had a penalty claim ignored when he went down in the area, while Sinclair Armstrong was kept out by Daniel Bachmann at the other end.