It was not only a match Bolton should not have lost on the balance of play, but one they should have won with something to spare. They had any number of chances, but a lack of steadiness, nerve and luck in front of goal left them open to the sucker punch.
When it arrived, five minutes from time from Peter Kennedy, it took Watford above Bolton on the table, which was a reward for their tenacity rather than for their share of the play.
Although both sides had a fistful of chances to opening the scoring in a frenetic 15 minutes, Graham Taylor's men spent most of the match on the back foot after that.
Nathan Blake, Arnar Gunn-laugsson and Ricardo Gardner had all gone close for Bolton before they made their breakthrough, Gunnlaugsson shooting low with his right foot past Alec Chamberlain for the 11th goal of his season.
Bolton's more cynical and world-weary followers have already seen enough this season to know that they cannot relax when their side takes the lead.
In theory, the return of the most experienced members of their defence, Gudni Bergsson and, for the first time this season, first-choice goalkeeper, Keith Branagan, should have made them more resilient.
In practice, after Blake's header had skimmed the bar, they were wide open to Watford's counter-attack, Kennedy getting in a hard, low cross and Gifton Noel-Williams hitting his close-range shot into the net.
Bolton still made all the running in the second half, coming close to regaining their lead on a number of occasions.
The most galling moment of all for their increasingly frustrated supporters came when Gunnlaugssen met Neil Cox's deep cross with a textbook downward header, which reared up to hit the underside of the bar and bounced clear.
Gunnlaugsson, irrepressible throughout and a player, who, more than most, did not deserve to be on the losing side, had a furious penalty claim turned down when he was sent tumbling and the skilful Gardner saw his shot flash past the angle of bar and post.
But just as Bolton thought they might have to settle for the draw, Watford came up with the killer blow, Micah Hyde getting to the byline on the right and planting a cross into the vacant section of the penalty area for Kennedy to volley home.
Bolton (4-4-2): Branagan; Cox, Bergsson, Fish, Whitlow; Johansen, Frandsen, Jensen, Gardner (Taylor, 84); Blake, Gunnlaugsson. Substitutes not used: Phillips, Hold-sworth.
Watford (4-4-2): Chamberlain; Gibbs (Gudmundsson, 80), Palmer, Page, Robinson; Bazeley, Hyde, Johnson, Kennedy; Smart (Mooney, 90), Noel-Williams. Substitutes not used: Wright.
Referee: R Pearson (Peterlee).