Hornets boss Aidy Boothroyd picked up October's gong on the back of five straight wins that had seen his side go into the first versus third clash with a six-point cushion at the top.
That gap barely suffered after Bristol City were held to a draw at Wolves, but the Hornets saw their ten-game unbeaten run end, suffer their first home league defeat and fail to score in the Championship for the first time this season.
This should prove to be no more than a blip for Watford, but Boothroyd would have been concerned by the manner in which his defence imploded during a crazy two-minute first-half spell.
The Hornets backline did admittedly have to regroup after losing Jay DeMerit to a ninth-minute calf injury, but they did not look unduly troubled during the opening 30 minutes which the home side marginally shaded from an attacking perspective.
The first mistake was committed by Jordan Stewart, whose lapse left Kevin Phillips with a clear run on goal.
Although Richard Lee did well to block his low shot, the ball rebounded back into the heart of the penalty area where Ishmael Miller fired home his ninth goal of the season through a packed goalmouth.
As Watford sought to recover from the setback, they all but gifted West Brom their second moments later.
Zoltan Gera was unlucky not to be scorer when he saw his shot on the turn from a Paul Robinson cross bounce down and out off the underside of the bar, but instead of reacting to the loose ball, the Hornets players just stood and watched as Phillips took advantage to join his strike-partner on nine for the campaign.
A bad afternoon for the league leaders got even worse four minutes after the restart when Jonathan Greening swung over a free-kick from the right and Martin Albrechtsen arrived unmarked to plant a free header inside Lee's right-hand post.
Apart from Darius Henderson's first-half header which Dean Kiely saved, the closest the Hornets came to scoring was when Adrian Mariappa's close-range stoppage-time effort was blocked.