Bradley Dack struck in extra-time as Gillingham came from behind to stun Premier League side Watford 2-1 and reach the third round of the EFL Cup.
Walter Mazzarri looked set to enjoy his first win as Hornets boss when Odion Ighalo came off the bench to fire them ahead before Dack missed a penalty.
But Mark Byrne struck eight minutes from the end of normal time to level the scores before Dack’s heroics, as Gillingham beat top-flight opposition for the first time since dumping Portsmouth out of this competition at the same stage in 2005/06.
The Hornets made nine changes from their defeat against Chelsea three days ago with only captain Troy Deeney and Adlene Guedioura keeping their places in the team while, for the visitors, Gills manager Justin Edinburgh named close to his strongest side.
But that did not seem to matter as it was the home side who enjoyed the better of the opening exchanges.
In the 13th minute, the Hornets should have taken the lead. Matej Vydra latched onto an inch-perfect pass from Abdoulaye Doucoure and subsequently back-heeled the ball into the path of the onrushing Ikechi Anya but the Scotland striker fired straight at Stuart Nelson.
At the other end, Cody McDonald had the first chance for the away side in the 18th minute. He did well to connect with Byrne’s cross but his looping header failed to trouble goalkeeper Costel Pantilimon.
Ighalo broke the deadlock in the 57th minute. The Nigeria striker, who scored 17 goals in all competitions last term, chased down Deeney’s pass over the top of the defence and got to the ball before the onrushing Nelson could collect and prodded home from all of five yards.
Gillingham were awarded a penalty in the 70th minute. Substitute Dack played in Elliot List with a perfectly-weighted through ball and saw his team-mate felled by Juan Carlos Paredes. Dack could not make it count though, as he blazed over the bar from 12 yards.
The writing looked on the wall but the Gills’ persistence was rewarded when they sensationally equalised in the 82nd minute with a magnificent goal from Byrne.
The winger gained possession 25 yards out and smashed the ball into the top left-hand corner. Dack then atoned for his penalty miss shortly into extra time.
Emmanuel Osadebe found him with a cross and Dack’s fizzing effort was initially pushed onto the post before the ball nestled over the line.
Gillingham manager Justin Edinburgh was rightly pleased with his side having lost 5-0 at Scunthorpe United on Saturday:
“In the end we deserved it. We had to ride our luck a little bit in the first half. After Saturday we were a little fragile.”
“When we missed the penalty I thought we missed our chance, but we showed great courage and spirit to keep going.”
“It’s a proud moment for the football club to come to a Premier League team with international stars and win. The level of fitness from my players was quite extraordinary.”
Watford line up: Pantilimon, Nyom, Hoban, Kabasele (Ighalo 56), Paredes, Guedioura, Watson, Doucoure, Anya (Zuniga 94), Deeney, Vydra (Capoue 66).
Subs not used: Gomes, Kaboul, Holebas, Sinclair.
Goal: Ighalo (57).
Gillingham line up: Nelson, Pask, Konchesky (List 66), Ehmer, Oshilaja, Wright (Oldaker 74), Knott, Byrne, Osadebe, McDonald, Emmanuel-Thomas (Dack 66).
Subs not used: Hadler, Hessenthaler, Donnelly, Dickenson.
Goals: Byrne (82), Dack (102).
Booked: Byrne (118).
Referee: Mr James Linington
Attendance: 7,004 (849 Gills)