Odion Ighalo came off the bench to net his 15th goal of the season with an 89th-minute winner that sent Watford past Nottingham Forest in the FA Cup.
The hosts had looked set to extend their unbeaten run to 14 games in all competitions in a contest where two much-changed sides mustered little at the City Ground.
However, Ighalo, called for with 18 minutes remaining, capitalised on Kelvin Wilson's error to fire home in a 1-0 victory that consigned Forest to their first defeat since November 21.
It was Wilson's inability to deal with Allan Nyom's right-wing cross that presented the Nigerian with the chance and it provided another low moment in this fixture for the centre-back, who was sent off in the last meeting between these two following a clash with Ighalo.
The two managers made 15 changes between them and there was a first appearance for Watford goalkeeper Costel Pantilimon following his switch from Sunderland earlier this month.
The giant Romanian was not eased into action, though, being pressured into making a shanked clearance out for a throw with his first touch in a moment which set the tone for a scrappy half.
Nordin Amrabat, another new recruit starting for the first time, wildly slashed at his opening attempt while ex-Forest midfielder Adlene Guedioura was wayward with his first two attempts from distance.
He did finally find his range to strike straight at goalkeeper Dorus De Vries, who then had to get down low to deny Ikechi Anya after he was found on the right flank by Troy Deeney.
Anya was alert to get in position to make that shot but he was completely unaware of Jamie Ward's presence on the stroke of half-time and it nearly cost the visitors.
The Scotland international was robbed by Forest's Northern Irish winger midway inside his own half and Ward ran through on goal before attempting to clip the ball beyond Pantilimon, only to see it roll wide of the mark with the last act of the opening 45 minutes.
Nine minutes after the interval the Hornets finally sparked into life when Amrabat's terrific volley from the edge of the box was brilliantly pushed away from the top corner of his net by De Vries.
The Morocco international was growing into the game and had another attempt stopped by De Vries, flicking Anya's right-wing cross at the keeper after the Scot had turned David Vaughan inside out.
Watford boss Quique Sanchez Flores called on Ighalo and Jose Manuel Jurado, while counterpart Dougie Freedman threw on Nelson Oliveira and went two up top in the closing stages to try and force a winner.
And it was Igahlo who delivered it with a clinical finish after Wilson's mis-kick from Nyom's delivery landed at his feet.
Forest almost grabbed an equaliser but Pantilimon used his frame to scoop the ball away from Oliveira as he tried to go around him.
Source : PA