James Collins came off the bench to score a penalty with his first touch as mid-table Luton Town earned a deserved derby victory over promotion-chasing Watford.
Second-placed Watford were outplayed before the break, but an improved second-half display looked like earning them a point which would have maintained their seven-point advantage over Swansea City, who fought back to draw with Wycombe Wanderers.
But with 12 minutes left Collins calmly struck the winner and, despite a frantic finish in which the Hornets had Kiko Femenia sent off and Andre Gray saw an equalising goal disallowed, the hosts held on.