Manuel Almunia's last gasp error handed Millwall an equaliser in the fifth minute of injury time after Almen Abdi had handed Watford an 86th minute lead.
Watford took the lead after 17 minutes when Lewis McGugan drilled home his ninth of the season from the edge of the 18 yard box. Steve Morison equalised for Millwall after 64 minutes.
Abdi restored Watford’s advantage with four minutes remaining, but in the fifth minute of injury time, Martyn Woolford grabbed a precious point for Ian Holloway’s side in fortuitous circumstances.
Beppe Sannino made two changes from the side which beat Leeds United on Tuesday. In came McGugan for Sean Murray and Tommie Hoban for the suspended Marco Cassetti.
The game began at a frantic pace, with Carlos Edwards venturing forward from right back to drive comfortably over from 30 yards after two minutes.
McGugan had Watford’s first chance after seven minutes, lashing a half volley narrowly wide of David Forde’s right-hand post after Ikechi Anya cleverly nodded into his path.
Shaun William responded for the Lions, nodding wide via a deflection at the far post.
Watford won a corner after 12 minutes, Gabriele Angella rising to flash a header wide of the far post from a McGugan delivery.
Good link up play from Deeney and McGugan should have resulted in a chance for Albert Riera, but instead McGugan attempted to round the covering Millwall defender, only for David Forde to rush out and collect.
McGugan opened the scoring two minutes later, drilling past Forde at the near post and into the bottom corner from the edge of the area.
The former Forest man tried his luck again a couple of minutes later, lashing wide from distance.
Deeney was next to test Forde, advancing into the Lions’ penalty area and drawing a save from the Irish international from an acute angle.
Another chance fell McGugan’s way when in the 24th minute he found the room to bend an effort wide of Forde’s left-hand post from 20 yards.
Manuel Almunia was forced into his first save after 27 minutes when he beat away a fierce drive from Lions’ left-back Scott Malone from the edge of the area.
Owen Garvan tried his luck three minutes later, turning and firing against a Watford body to earn the hosts a corner.
The Lions had their best chance in the 33rd minute when Lee Martin robbed Joel Ekstrand and fed Woolford, who in turn cut the ball back for Garvan, who was denied by Almunia from a central position.
Deeney and Garvan both had chances for their respective sides before the break, neither player stretching the opposing goalkeeper.
Millwall substitute Morison had the Lions’ first attempt of note after the break, flicking Edwards’ throw on comfortably into the palms of Almunia.
Stefan Maierhofer should have equalised for the Lions in the 55th minute, rising unchallenged to head Martin’s free-kick comfortably over from six yards.
Millwall were rewarded for a brighter start to the second half when Morison controlled a corner which had been flicked on to the back post and crashed a half-volley past Almunia from the angle of the six yard box.
Substitute Fernando Forestieri brought the best out of Forde in the 70th minute, racing onto a ball from fellow sub Daniel Pudil and testing the Lions’ keeper with a firm left footed drive.
Abdi tried his luck with the subsequent follow up and was denied, the Swiss again coming close a minute later with a blocked shot from inside the penalty area.
Millwall skipper Alan Dunne forced Almunia into an excellent save with five minutes to go, the Spaniard pushing away a 20 yard piledriver which appeared destined for the top corner.
The Hornets broke almost immediately from the corner and after working the ball across the face of the Lions’ goal, Abdi steered into the bottom right-hand corner via deflection to put Sannino’s side back in front.
Almunia again did well to deny the Lions in the 90th minute, getting down well to push Woolford’s firmly struck effort around the post for a corner.
In the final minute of five added on, Woolford finally got the better of Almunia to equalise for Millwall. The winger could not believe his luck as his left-footed half-volley squirmed past Almunia at the near-post to earn the Lions a draw.
Millwall: Forde; Edwards, Dunne (c), Beevers, Malone; Woolford, Williams, Garvan (Morison HT), Bailey, Martin (Jackson 62); Maierhofer (Easter 74).
Not used: Bywater (GK), Robinson, Fredericks & Abdou.
Watford: Almunia (c); Hoban, Ekstrand, Angella; Faraoni, Abdi, Tozser, McGugan (Murray 83), Riera (Pudil 67); Deeney & Anya (Forestieri 45).
Not used: Bond (GK), Belkalem, Merkel & Ranegie.
Bookings: Riera for a foul on Edwards (3); Martin for throwing the ball away (26); McGugan for kicking the ball away (36); Murray for a foul on Easter (84).
Attendance: 10,851 (1369 Watford fans).
Referee: Mr K Stroud.