Watford boss Javi Gracia saluted Troy Deeney after the striker floored West Ham and a corner flag.
The Hornets skipper slammed home a first-half penalty and Gerard Deulofeu added a late second to stun West Ham 2-0 and scupper their bid to win five matches on the spin.
Deeney, relishing his role as pantomime villain at the London Stadium, celebrated his successful spot-kick by punching the corner flag out of the ground and goading the home supporters.
The 30-year-old certainly seems to enjoy playing against West Ham. Two seasons ago he accused the Hammers of "trying to mug us off" with their showboating after he inspired Watford to come from two goals down and win 4-2.
Fortunately only one home supporter reacted to his 'exuberant' celebration, and the paper cup that was thrown towards the striker fell a long way short.
Gracia said: "What I saw was that after the goal he was running to the corner. I don't know what he said. I can't tell you if it's a good or bad celebration, I often see players running into the corner.
"I like the way he plays, his commitment, all the things he does day by day. Like all the players he maybe makes mistakes but he is our captain and an important player for us."
Deeney's big moment arrived on the half hour, when Robert Pereyra wriggled free in the area only to be caught by a clumsy late challenge from Fabian Balbuena.
Lukasz Fabianski had saved Deeney's previous three penalties but he had no chance with the fourth, the frontman blasting into the top corner.
West Ham had chances to equalise but Michail Antonio twice hit the woodwork before Deulofeu finished them off three minutes from time.
"I think we had a good performance. We knew playing against West Ham, in this good moment for them, that it would be demanding and it was like we thought," added Gracia.
Hammers boss Manuel Pellegrini has been targeting a top-six finish but it is now Watford who are suddenly threatening the European spots.
"Really I think we were a bit unlucky to not at least draw," said Pellegrini.
"The penalty was the only action in our box in the first half, and we had three or four chances to draw the game. Unfortunately we didn't score and then they scored their second.
"We had to be faster and more accurate. In the second half they had a couple of chances but before that we had all the clear opportunities to draw."
