Substitute John-Joe O'Toole rescued Watford with an extra-time volley to seal a 2-1 win over Darlington in the Carling Cup second round.
Damien Francis was on target on 37 when a flowing move was cleared out to Ross Jenkins whose low drive struck a Darlo defender and fell into the midfielder's path.
Darlington goalkeeper Simon Brown was already committed to his dive leaving Francis with a tap-in and he made no mistake from eight yards.
In the second half Darlington couldn't break through until stoppage time.
Brown hoofed a clearance down the pitch and Gregg Blundell collected it before chipping Scott Loach.
The game appeared to be drifting towards a penalty shoot-out before O'Toole struck with four minutes remaining.
Lewis Young crossed for Liam Henderson who chested the ball before O'Toole crashed a half-volley into the the net.