Millwall's magnificent 22-match unbeaten home run was ended in emphatic style as Watford sent The Lions spinning to their heaviest defeat in eight years.
Not since the opening home hammering by Rotherham United in 2002 have a team smacked The Lions for six in their own backyard.
Indeed the last team to smuggle three points away from SE16 was Wycombe Wanderers on 21st November 2009 - but it was The Hornets who dished out a Championship reality check of the highest order.
It proved to be a miserable afternoon for Jackett, facing the team he made his name for, for the first time as a manager.
"We contributed to our downfall, we were poor," said Jackett.
"Watford played very well and at a high tempo, but we didn't match them. We didn't play our game and that is disappointing."
The Hornets had failed to taste defeat on their travels in the league this season and they showed just why with a powerful performance to which The Lions had no answer.
Watford out-passed, out-fought and out-thought a listless Lions who simply failed to get out of the starting blocks in a nightmare 90 minutes during which if it could go wrong, it did go wrong.
The Hornets cruised into a 2-0 lead with 13 minutes on the clock and never looked back.
John Eustace steered home Don Cowie's ninth-minute corner for the first and then Jordan Mutch fired past keeper David Forde at the second attempt four minutes later.
Marvin Sordell could have made it three on 20 minutes, only to be denied by Forde after the visitors sliced through the home rearguard once more.
At the other end Steve Morison rattled the underside of the crossbar from an acute angle, but The Lions rarely troubled Watford keeper Scott Loach.
The Hornets effectively killed the game as a contest deep in first-half injury-time, Sordell latching onto the loose ball following a Mutch drive.
Jackett brought on Chris Hackett and Theo Robinson, for his Lions debut, at the start of the second period in place of James Henry and Neil Harris and although The Lions tried to find a way back into the game it was Watford who put the result beyond doubt, Adrian Mariappa heading home from another corner.
Although Liam Trotter pulled a goal back by sweeping home seconds after the restart, The Hornets added further goals through Danny Graham and Martin Taylor.
Taylor's goal perhaps best summed up the afternoon for Millwall; Hackett's attempted clearanced was sliced over his own head for a corner - and Taylor rose highest to guide the ball past Forde, the third of three set-pieces that had beaten The Lions.
Millwall: Forde, Dunne, Barron, Abdou (Mkandawrire, 76mins), Robinson, Ward, Schofield, Trotter, Morison, Harris (T.Robinson, 46mins), Henry (Hackett, 46mins). Subs not used: Mildenhall, Lisbie, Craig, Smith, T.Robinson, .
Bookings: Barron (49mins), Morison (73mins)
Scorer: Trotter (57mins)
Watford: Loach, M.Taylor, Graham, Cowie, Doyley, Mutch, Eustace, Sordell (Deeney, 67mins), A.Taylor (Hodson, 69mins), Buckley (McGinn, 60mins), Mariappa. Subs not used: Gilmartin, Bennett, Walker, Jenkins.
Bookings: Doyley (30mins), Sordell (55mins)
Scorers: Eustace (7mins), Mutch (13mins), Sordell (45+2mins), Mariappa (54mins), M.Taylor
Ref: Neil Swarbrick* (* change to RL Shoebridge)
Att: 12,562 (away 1,017)