Three goals in a devastating 30 first-half minutes sent Millwall sliding to their heaviest defeat - and only second loss in 14 games - at Hartlepool United.
It was an afternoon best forgotten for Kenny Jackett's men who created some decent chances in front of goal, but saw some sloppy defending capitalised on in clinical fashion by their hosts.
Last season Jackett's side managed to overturn a 2-0 half-time deficit and went on to record a sensational 3-2 win thanks to a splendid second-half hat-trick from super-sub Neil Harris.
But trailing 3-0 after just 29 minutes this was always going to be a tough mountain to climb, and one that ultimately proved a mission impossible as The Lions gamely tried, but were unable, to undo the first-half damage that had been done.
After a quiet opening, it had all looked so positive for The Lions as Steve Morison thumped the woodwork early on after closing down a clearance from keeper Scott Flinders.
Even when Peter Hartley drilled home the opener against the run of play on 12 minutes from 18 yards, Millwall threatened with Morison again denied when his header from Chris Hackett's cross virtually from the restart smacked the bar.
Leon McSweeney, a late replacement for Ritchie Jones forced a smart save from keeper David Forde on 19 minutes with a crisp effort that saw the ball pushed round the post for a corner.
Millwall were left riding their luck shortly after when Bjornsson's goalbound header was blocked on the line by Alan Dunne before Forde smothered the ball as it looked to be squirming into the back of the net.
But there was nothing lucky about Pools' second goal as Adam Boyd latched onto Ritchie Humphreys header, strode forward and then rounded Forde before calmly stroking home.
Having urged his side to show their mettle defensively after shipping 11 goals in their previous eight games, this was exactly the sort of start Lions boss Jackett did not want.
And things went from bad to worse when, with less than half an hour on the clock, Bjornsson made it 3-0 as from Neil Austin's cross as The Lions back four were again caught horribly cold.
Although The Lions had twice rattled the woodwork through Morison, they went in at the break knowing it was going to take a second-half miracle to turn this game around.
Not surprisingly changes were made at the start of the second-half as Millwall brought on Lewis Grabban and Marc Laird for Hackett and Bolder respectively.
Grabban was moved into a more direct forward role to support Morison, so often isolated during the opening 45 minutes, with Laird adding some attacking impetus to the midfield.
A decent cross from James Henry was just too far for Darren Ward to connect with and then an almighty scramble saw Grabban and Andy Frampton thwarted as The Lions tried to get a lifeline early in the second half.
But some resolute home defending continued to frustrate Millwall as they tried in vain to reduce the deficit.
Grabban was twice denied, Henry sent an effort over the bar but ultimately this sadly proved a task too much for The Lions, who never really got into any sort of fluid attacking rhythm throughout.

Hartlepool United: Flinders, Austin, Collins, Hartley, Humphreys (Clark, 82mins), Liddle, Jones, Monkhouse, McSweeney (Fredriksen, 70mins), Boyd (Greulich, 82mins), Bjornsson. Subs not used: Cook,Cherel, Sweeney, Rowell,.
Scorer: Hartley (12mins), Boyd (22mins), Bjornsson (29mins)

Millwall: Forde, Dunne, Frampton (Smith), Robinson, Ward, Bolder (Laird, 46mins), Schofield, Hackett (Grabban, 46mins), Henry, Abdou, Morison. Subs not used: Sullivan, Grimes, Fuseini, Price.
Booking: Morison (43mins)

Ref: Jock Waugh
Att: 3,153 (away 261)