Abdoulaye Doucoure secured a 2-2 draw for the Hornets with a blatant handball in the final minute. The French midfielder stooped to nod Deeney's header back across goal into the net - but the ball missed his head and he swept it home with his hand.
BBC pundit Martin Keown said Doucoure had "done a Maradona", and even Watford skipper Deeney admitted his side were fortunate.
The new Video Assistant Referee system has had a fairly gentle introduction in the past week, but it would have been the centre of attention at Vicarage Road.
"If VAR had come in we probably wouldn't get that but today we got it. It's the luck of the draw," Deeney told Sky Sports.
Yet how referee Roger East and linesman Derek Eaton missed Doucoure's sucker punch was a mystery.
Saints manager Mauricio Pellegrino was remarkably restrained given the decision had cost his side a first Premier League win in 10 matches.
"My feeling is like the players, we are a little bit sad," he said. "The last goal is something that surprised me because in the Premier League the level of referees is good. It's clear (handball).
"Everybody makes mistakes but the problem is at this level you have a lot to play for so we are disappointed.
"From the bench it is more difficult to see but the referee and the linesman are closer. I imagine the referee is not happy now."
A sheepish Marco Silva, the Watford boss, admitted: "I don't need to talk about the goal. I think everybody can see. It's something that can happen."