Ryan Jarvis bagged a hat-trick as Orient capitalised on a defensive horror show to inflict Millwall's first home defeat of 2007 in a ding-dong Den derby.
In what can best be described as a bizarre game, the visitors romped into a 3-0 lead with just 12 minutes on the clock and The Lions looked dead and buried.
But a spirited fight-back saw a Gary Alexander own goal and Poul Hubertz effort reduce the arrears.
And just as Millwall started to look capable of pulling off a sensational comeback with a third goal, two further howlers handed The O's the chance to put the result beyond doubt.
The Lions went into the game on the back of three successive victories, but it looked to be game over after 12 minutes as Alexander was left unmarked to head home a corner before Ryan Jarvis capitalised on two schoolboy errors in the home defence to give the East Londoners a lead they could scarcely believe.
Then before the game was 20 minutes old, both Neil Harris and Mark Phillips headed against the bar as The Lions plowed forward in search of some hope. When Danny Senda crashed a stunning free-kick onto the bar yet again five minutes after the re-start all hope seemed to be lost.
But Willie Donachie's men thought otherwise and when Alexander headed a Dave Brammer free-kick into his own net on 65 minutes it was game on.
Sub Poul Hubertz added a dramatic second shortly afterwards and the momentum had swung Millwall's way.
However, another howler, this time by Lenny Pidgeley let the O's off the hook, when his woeful goal-kick went straight to Jarvis who finished well as the keeper attempted to atone for his error.
The fifth goal from James Walker in injury time was academic as The Lions were left to rue too many costly mistakes.

Millwall: Pidgeley, Senda, Craig (Zebroski, 76mins), Elliott, Phillips, Shaw, Brighton (Hackett, 46mins), Brammer, Harris (Hubertz, 37mins), Byfield, Williams. Subs not used: Day, Robinson.
Bookings: Williams (15mins), Craig (48mins), Hubertz (60mins)
Scorers: Alexander (66mins og), Hubertz (69mins)

Leyton Orient: Garner, Barnard (Corden, 19mins), Lockwood, Saah, Mackie, Easton, Jarvis (Walker, 89mins), Alexander, Tudor (Tann, 75mins), Miller, Chambers. Subs not used: Morris, Demetriou.
Bookings: Garner (18mins), Alexander (39mins), Tann (84mins), Corden (86mins)
Scorers: Alexander (5mins), Jarvis (7, 12, 76mins), Walker (90mins)

Ref: Alan Wiley
Att: 10,356 (Away 1,060)