Ricardo Scimeca's late blunder gifted Watford the win against visiting promotion rivals Cardiff City.
Ricardo Scimeca's ill-judged back-pass gifted Watford a win
The Bluebirds had been second best for much of the match but only fell behind when Malky Mackay, who had earlier hit the bar, headed home after 69 minutes.
However, Jeff Whitley's left-footed volley, after Jason Koumas' good work, looked to have rescued Cardiff.
But Marlon King intercepted Scimeca's back-pass to score Watford's winner, although Koumas then hit the post.
Watford manager Adrian Boothroyd:
"There is an ingredient that makes winners, it's not luck, and Malky Mackay is one.
"It was naivety on our part that let Cardiff back in and things went on out there that we can learn from.
"We're up with the big clubs. There is an expectation now which is incredible seeing where we've come from, but I'll make sure we don't get too euphoric."
Cardiff manager Dave Jones:
"We fought hard to stay in the game, then we fought hard to get back into the game, and then it comes down to one mistake.
"When we got the equaliser we could put our foot on the ball, but he didn't put enough weight on a backpass.
"Ricardo's frustrated but no-one will blame him - it happens. While there are still points on the board we'll keep going."
Watford: Foster, Doyley, Mackay, DeMerit, Stewart, Eagles (Chambers 90), Mahon, Spring, Bouazza (McNamee 83), Young, King. Subs Not Used: Chamberlain, Carlisle, Diagouraga.
Goals: Mackay 69, King 88.
Cardiff: Alexander, Barker, Cox, Purse, Ledley, Whitley (Weston 85), Ardley, Koumas, Scimeca, Jerome, Thompson. Subs Not Used: Boland, Loovens, Margetson, Ndumbu-Nsungu.
Booked: Scimeca, Thompson.
Goals: Whitley 77.
Ref: C Penton (Sussex).