Not since January 2004, when Crystal Palace won 5-1 at Vicarage Road, have the Hornets lost by a four-goal margin at home, although it might have been a different story had a 16th-minute 25-yard free-kick from Don Cowie dipped under, rather than crashed off the crossbar.
But after that Watford were the architects of their own downfall not once, but twice before the interval.
The Bluebirds were gifted the opportunity to take the lead in the 24th minute when Craig Cathcart pulled back Jay Bothroyd in the area and Peter Whittingham comfortably converted from 12 yards.
Worse was to follow four minutes before the break when Cardiff scored a bizarre second.
There appeared to be no danger when Adam Matthews launched a free-kick forward from just inside the Hornets half, but the ball caught the wind and keeper Scott Loach misjudged the flight was left red-faced as it sailed over him to secure a 2-0 interval lead.
The manner of the second goal may have been fortuitous, but it was deserved as in the five minutes beforehand Michael Chopra had inexplicably fired wide with the goal at his mercy and Mark Hudson then hit a post with a header.
Watford did mount a rally, but only after the visitors had wrapped up the points with two goals in as many second-half minutes.
First, the Watford defence stood and admired as Whittingham found space in the area to head Gavin Rae's ball back into the area inside Loach's left-hand upright from around eight yards.
Then Bothroyd finished emphatically with a shot on the turn after being given time and space in the 18-yard box.