Jose Mourinho and his Manchester United side endured a week to forget as Watford secured a well-deserved victory over their Premier League rivals.
Going into the game on the back of a derby defeat at home to Manchester City and a Europa League loss at Feyenoord, United were largely disappointing as Mourinho slipped to three successive defeats as a manager for the first time in over a decade.
The in-form Etienne Capoue struck his fourth goal in six games to put the hosts ahead at Vicarage Road and although Marcus Rashford levelled on 62 minutes with United's first shot on target, Camilo Zuniga came off the bench and scored almost immediately.
53 - Juan Zuniga scored just 53 seconds after coming on as a substitute. Impact.— OptaJoe (@OptaJoe) September 18, 2016
A late Troy Deeney penalty secured a memorable 3-1 win for the Hornets - their first victory over United since 1986.
No Manchester United player has scored more goals than Rashford since he burst onto the scene in February and his more esteemed team-mates were largely poor, with skipper Wayne Rooney and the world's most expensive player, Paul Pogba, off-colour in the midfield and Zlatan Ibrahimovic well-marshalled by the Watford defence.
Watford passed up a gilt-edged chance to open the scoring as miscommunication between David de Gea and Chris Smalling saw the ball fall to the feet of Odion Ighalo but, with the goal gaping, he slid his effort wide.
Only a stunning De Gea save kept out Deeney's header in the 16th minute as the Hornets continued to swarm around the United defence.
The visitors should have made Watford pay for their profligacy as Ibrahimovic started a lightning counter-attack which he failed to finish off as Rashford's deflected low cross was thrashed into the side-netting by the Swedish forward.
After a delay in proceedings following head injuries for Daryl Janmaat and Anthony Martial, Pogba came within inches of scoring his first United goal as he arrowed a shot from 25 yards which cannoned back off the crossbar.
But it was Capoue, so sharp in front of goal so far this season, who struck from Janmaat's cross after Miguel Britos had robbed Martial of possession on the edge of the United box, a challenge which irked Mourinho, who wanted a free-kick awarded.
Martial, seemingly unable to recover from a mixture of both the knock to his head and the challenge from Britos which led to the goal, was replaced by former Watford favourite Ashley Young.
United still had not been at the races but they were level just after the hour mark as Ibrahimovic's cross was blocked into the feet of Rashford, who reacted quickly to flash home.
The visitors took control from that point and only a tremendous stop from Heurelho Gomes prevented Ibrahimovic heading home Marouane Fellaini's cross with 12 minutes remaining.
But it would be Watford celebrating as Walter Mazzarri brought on Zuniga for goalscorer Capoue, and the Napoli loanee needed just seconds to slam home.
Deeney added the third from the spot in the fifth minute of stoppage time after Zuniga was upended by Fellaini.