Watford recorded a spectacular win over Manchester United this afternoon, to secure their first victory over the Red Devils for the first time in 30 years.
Etienne Capoue opened the scoring with a well-taken first-time strike to ensure the Hornets went into the half-time break a goal to the good.
Marcus Rashford drew the scores level with an instinctive finish, but it would be Watford who went on to secure all three points.
Substitute Camilo Zúñiga proved to be the difference for the Hornets. He scored a fine goal at the death, before winning a penalty which Troy Deeney smashed past De Gea.
The win sees the Hornets rise into the top half of the Premier League table, with seven points from a tricky opening five games.
The Watford team had one change from the side which won at West Ham United last weekend.
Younes Kaboul missed out and was replaced in a straight switch with Sebastian Prödl in the back-three.
The early exchanges suggested that it would be an open affair, with United fashioning the first opening as Chris Smalling headed Wayne Rooney's free-kick just wide of Heurelho Gomes' post.
The Hornets enjoyed a promising spell of play and were inches away from taking the lead on ten minutes, as Holebas swung a brilliant free-kick into the box. Prödl steered a header towards goal but it just flashed wide of the far-post.
Odion Ighalo should have put the Hornets ahead just a couple of minutes later. De Gea jumped for a high ball into the box but clashed with Smalling, spilling the ball into the path of the Watford front-man. He hit his effort first time, but it flew frustratingly wide of the post.
The United shot-stopper proved his quality shortly afterwards. He pulled off an excellent finger-tip save to deny Deeney's firm header from Daryl Janmaat's powerful ball deep into the box.
United had an excellent opportunity of their own as Marcus Rashford broke down the left before feeding a great low pass into the box. Zlatan Ibrahimovic raced in to meet the ball, however the Swede's low strike brushed the side netting and went out of play for a goal-kick.
The crossbar came to the Hornets' rescue just past the half hour mark as United's world record-signing Paul Pogba collected a ball outside the box. He opened his body before curling an excellent effort towards goal. It dipped at the final moment but cannoned back off of Gomes' bar.
Not long after, Rooney hit an out-swinging corner in from the right which Smalling met with a header, but his effort went wide once more.
Minutes later the Hornets stormed into a deserved lead. Miguel Britos did excellently to win the ball off Martial deep in United's half before sliding the ball in to Janmaat.
The full-back pulled an excellent ball back to Etienne Capoue who steered a first-time effort into the bottom corner via the boot of De Gea, for his fourth of the season.
The Hornets had to remain alert as Prödl did well to block Luke Shaw's low shot at goal.
And the Austrian did brilliantly again as he denied Ibrahimovic a shot at goal, with a last-ditch sliding tackle - after he had been played in by a loose header from midfield.
United's front-man was again denied as he seemed destined to take a shot at goal. This time it was Gomes who did well to clip the ball away from the striker's feet after a poor back-pass.
On the stroke of half-time Capoue was brought down outside the United area. Jose Holebas lined up a free-kick which he guided towards De Gea's left, but the shot-stopper held comfortably.
The second-half started in similar fashion to the first as United mustered up the first opportunity. A ball into the box fell to Ibrahimovic but he dragged a first-time volley well wide.
Mazzarri made his first change of the afternoon as he brought on Nordin Amrabat for Janmaat on 50 minutes.
Capoue's corner into the box, just shy of the hour mark, fell to Amrabat on the edge of the box. He hit a first-time effort at goal which Rooney dived in-front of to block wide for a corner.
United were on level terms though just past the hour mark. Ibrahimovic's ball into the box from the left deflected off Behrami and fell to Marcus Rashford who had the simple task of tapping-in from only a couple of yards out.
The full-back played an excellent ball into the box which Roberto Pereyra flicked on into the danger-area. It caused chaos as De Gea first came to clear, after Shaw had sliced an attempted clearance into the air. However the ball just wouldn't drop to a yellow shirt, and it was eventually cleared.
Gomes pulled off an excellent save to his right to deny Ibrahimovic what seemed a certain goal. The striker guided a header towards the corner from a Marouane Fellaini cross, but the keeper darted to his left to tip the ball away.
Mazzarri made his second change of the match with ten minutes to go as Camilo Zúñiga came on in place of goalscorer Capoue in the Hornets' midfield.
And what a change that proved to be.
Pereyra pulled the ball after he had been slid in down the right, and Zúñiga showed excellent composure to guide a first-time right-footed effort into the bottom corner past De Gea, Vicarage Road was delirious.
Mazzarri made his final substitution of the match as Isaac Success came on in place of the lively Ighalo, with just stoppage time remaining.
Success broke down the left, before playing Zúñiga into the box. He was brought down by Fellaini and referee Michael Oliver pointed to the spot.
Captain Deeney stepped up to take the kick, thumping his effort emphatically past De Gea to secure all three-points.
The full-time whistle followed moments later as the Hornets secured back-to-back wins and their first win over Manchester United in thirty years.
HORNETS: Gomes (GK); Cathcart, Prödl, Britos; Janmaat (Amrabat, 50), Pereyra, Behrami, Capoue (Zúñiga, 81), Holebas; Deeney (c), Ighalo (Success, 90).
Subs not used: Pantilimon (GK), Kenedy, Watson, Guedioura.
Goals: Capoue (34), Zúñiga (85), Deeney (90)
Yellow Cards: Britos (77), Holebas (77), Deeney (86)
UNITED: De Gea (GK); Valencia (Mata, 62), Smalling, Bailly, Shaw (Memphis, 85); Pogba, Fellaini; Rashford, Rooney, Martial (Young, 38); Ibrahimovic.
Subs not used: Romero, Blind, Carrick, Herrera.
Goals: Rashford (63)
Yellow Cards: Pogba (75), Fellaini (88), Memphis (89), Rooney (90)