John-Joe O'Toole came off the bench to rescue Watford with the winner deep into extra time to sink Darlington.
Late winner from O'Toole downs Quakers
John-Joe O'Toole came off the bench to rescue Watford with the winner deep into extra time to sink Darlington in the Carling Cup.
Damien Francis' first-half strike had put the Hornets on course to progress, but the League Two side were in charge in the second half and grabbed a last-gasp equaliser.
Forward Gregg Blundell beat the offside trap to latch onto goalkeeper Simon Brown's long clearance and cleverly lob Scott Loach from the edge of the box.
The visitors then went close to taking the lead in extra-time before O'Toole smashed in from an acute angle to put the Hornets in the third-round draw.
The Hornets romped to an 8-0 victory when these sides met at this stage of the competition 21 years ago but the Quakers never looked like capitulating this time and were unlucky to go out.
They controlled the majority of the second half but failed to really test Loach in the Watford goal until the last minute when he brilliantly denied Billy Clarke.
Lewis Young - younger brother of Aston Villa's former Hornet Ashley Young - was handed his first start as Boothroyd made eight changes from the weekend defeat at Nottingham Forest.
Rattled
Boothroyd's side nearly took the lead in the 15th minute. Tamas Priskin burst into the area and stood up a ball to the far post where Young was haring in but the 18-year-old rattled his volley into the legs of Brown when he should have scored.
Will Hoskins then smashed a half-volley over from the edge of the area before Francis struck in fortunate circumstances.
A flowing move was cleared out to 17-year-old Ross Jenkins whose low drive struck a Darlington defender and fell into Francis' path.
Brown was already committed to his dive leaving the former Norwich midfielder with the simplest of tap-ins and he made no mistake from eight yards.
The second-half began at a far slower pace as the Championship side looked to take the sting out of the match.
But instead it was Dave Penney's side who wrestled control of the match with Jason Kennedy twice denied by last-ditch tackles in the area and Ricky Ravenhill shooting wide when well-placed.
Captain Stephen Foster then shot wide after Watford made a mess of clearing a free-kick before Clarke created a chance out of nothing.
The youngster was surrounded by defenders but wriggled free into space and sent in a low curling shot which looked destined for the far corner until Loach tipped it wide.
Hoofed
Any chance of a comeback looked to have gone but the Quakers scored from nothing in stoppage time.
Brown hoofed a clearance down the pitch and Blundell was somehow allowed to collect it and run in to chip the advancing Loach.
There were suspicions of offside about the goal but the Quakers started extra-time in the ascendancy and substitute David Poole stung the palms of Loach with a rasping drive.
Leigh Bromby then made a mess of another long punt downfield but Clarke - who was excellent throughout - lost his composure and sliced his shot over.
Darlington were now cutting through at will and only a tremendous saving tackle from Mat Sadler prevented Kennedy from scoring.
The game appeared to be drifting towards a penalty shoot-out before O'Toole struck with four minutes remaining.
Young lifted a cross into the area where fellow youngster Liam Henderson controlled the ball on his chest.
The ball bounced high and O'Toole took the ball off his team-mate before crashing a half-volley into the roof of the net.
