It was probably the Foxes supporters who left for home the happier after Dany N'Guessan's stoppage-time header salvaged a point for the visitors.
But, by the same token, they also had every right to feel disappointed because an hour earlier their team had been 2-0 up and seemingly on course for a first away win of the season.
It was in-form Matty Fryatt who did the damage in the first half, scoring twice to take his tally to five goals in three matches and eight for the season.
First, the striker tucked away a penalty after John Eustace was adjudged to have handled a Robbie Neilson cross, and then Fryatt ran onto a Paul Gallagher throughball and turned Craig Cathcart before sliding the ball beyond keeper Scott Loach.
Fryatt didn't realise at the time though, but all he'd done was set the stage for a remarkable homecoming for a Hornets hero.
Heidar Helguson scored 64 goals in five-and-a-half years for Watford before leaving four years ago, but on Tuesday he returned to the club where he made his name on loan from Queens Park Rangers.
Having almost scored with his first touch, the 32-year-old caused sufficient havoc in the box to force Jack Hobbs to head a Lee Hodson ball into the path of Danny Graham.
The former Carlisle United frontman then turned his man and did well to prod the ball past keeper Chris Weale with the outside of his right foot to score his fifth of the campaign.
That goal came in the 58th minute and, within two minutes, Watford were level.
Hodson was again the supplier, this time with a cross from the right that was deflected across the six-yard box that presented Helguson with a gift to celebrate his second 'debut' and 200th appearance for the Hornets.
The sense that the script was written was completed in the 77th minute when Hodson again picked out Helguson and the former Fulham and Bolton Wanderers striker pulled off his man to beat Weale at his near post.
It completed a remarkable turnaround, but it was Leicester who had the last laugh when N'Guessan was left free in the area to head home a cross from the left.