After being the victims of the now infamous own goal that never was farce against Reading at the weekend, there was more than a hint of irony that the decisive action of the game should be perpetrated by Hammers midfielder Hayden Mullins, who unwittingly put through his own net after keeper Jan Lastuvka had made a complete hash of dealing with a second-half free-kick.
But it was no more than the home side deserved. Despite having 12 players, including nine recognised first-teamers unavailable through injury, they went toe-to-toe with their more illustrious opponents throughout and their youthful enthusiasm ultimately prevailed.
Both sides had reasonable chances in the opening period, but the calibre of these improved after the break.
Scott Loach appeared to do very well within two minutes of the restart to turn a close-range effort onto the post and behind, before a blockbuster from Watford left-back Jordan Parkes appeared to be going in until it was deflected off target.
However, it took a fantastic stop from Lastuvka to prevent the Hornets from taking a 59th minute lead as he showed great reactions to keep out a Smith header from close range following a Lee Williamson cross.
Within 11 minutes though, the Czech keeper had gone from hero to zero after completely failing to cut out Williamson's ball in from the right and the helpless Mullins could do nothing but turn the ball into his own net.
Within two minutes though, the Hammers missed their best chance to equalise when a deep ball in from the right picked out the unmarked Matthew Etherington, but he headed back across goal and wide.
Matthew Upson was also perhaps slightly unfortunate to see a late header from a corner rebound off the crossbar, but by that stage it was desperation stuff from the visitors.