Adam Virgo headed a 73rd-minute winner as struggling Brighton produced a classic smash and grab raid to condemn Millwall to a second home defeat in three games.
Virgo's goal was a sickening blow for Kenny Jackett's side, who were left wondering just how they had failed to win - and win comfortably - against an Albion outfit that parted company with manager Micky Adams just hours before kick-off.
It was a dream result for temporary Seagulls bosses Dean White and Bob Booker, who probably could not quite believe their luck as The Lions lack of that all-important killer touch in front of goal proved costly.
Millwall boss Jackett handed a debut to loan signing Vincent Pericard - one of three changes to the starting line-up against Swindon on Tuesday night - with Scott Barron replacing the suspended Andy Frampton and Dave Martin returning from a one-match ban to take his place on the left wing in place of the injured Lewis Grabban.
Striker Izale McLeod was ruled out after sustaining an injury in training on Friday so Pericard was partnered by Neil Harris up front.
And it took just under two minutes for the home side to test the Albion defence when Barron tried to slip the ball through to Pericard, but it flew agonisingly away from the striker's right boot as he was about to connect.
Adam Bolder's low shot on eight minutes was parried at full stretch by visiting keeper Michel Kuipers, only for Harris to lash the loose ball wildly over the bar from eight yards.
The Lions kept the pressure on and a great ball from Harris picked out Pericard on 11 minutes. The Cameroon international did well to work some space, but was then denied by keeper Kuipers who did well to block both the initial shot and then follow-up.
Kuipers certainly earned his crust in the opening half particularly, and he produced a magnificent save on 21 minutes; a slick move down the left saw Barron's delicious delivery pick out Harris and when he connected with a powerful header that looked destined for the top corner, there was Kuipers to somehow fingertip the effort away for a corner.
James Henry and Bolder, twice, went close from distance as MIllwall tried to make the all important breakthrough.
But with virtually their first penentrative attack during the first 45 minutes, it was Brighton who wasted a glorious opportunity when Chris Birchill somehow contrived to miss an empty net from five yards, and instead hit a shot that was more of a danger to Lions fans in Block 9.
Having produced an energetic display for 45 minutes, most supporters amongst a 9,226 Den crowd would have hoped and expected The Lions to push on and secure victory after the break.
Pericard saw his 46th-minute low strike held by Kuipers as Millwall looked to have picked up where they left off.
Pericard was inches away from marking his debut with a goal on the hour mark. Harris played the ball to Bolder, who in turn produced a defence-splitting pass for Pericard. The big frontman used his strength to charge between two Brighton players, took a couple of touches and then drilled an angled shot that flew no more than six inches wide of the far post.
Millwall began to flag as the game wore on, and you could sense that if Brighton were a little more adventurous, they could possibly just nick it.
Nicky Forster's 68th-minute looping header dropped onto the roof of the net was a timely reminder that The Lions could not afford to take their foot off the gas.
However, Jackett's men failed to heed the warning and five minutes later when Millwall could only half-clear a free-kick, Jason Jarrett's high, hooked return pass was headed home by the unmarked Virgo.
Jackett introduced Ashley Grimes and Marc Laird for Martin and Jimmy Abdou, but The Lions frustratingly could not really work up a head of steam.
Other than a Bolder shot on 82 minutes that flew a foot wide, it was a most disappointing finish for Millwall and the game petered out with a whimper from The Lions - and a surprise win for the visitors.

Millwall: Forde, Duffy (Hackett, 59mins), Barron, Bolder, Whitbread, Craig, Martin (Grimes, 75mins), Abdou (Laird, 75mins), Harris, Pericard, Henry. Subs not used: Pidgeley, Dunne.
Bookings: Abdou (59mins), Whibread (75mins), Craig (89mins)

Brighton & Hove Albion: Kuipers, Whing, El-Abd, Cox, Jarrett, Forster, Birchall, McNulty, Fraser (Hinshelwood, 84mins), Virgo, Elphick. Subs not used: K.McLeod, Sullivan, Hawkins, Carole.
Booking: Jarrett (31mins)
Scorer: Virgo (73mins)

Ref: Rob Styles
Att: 9,226 (away 1,111)