Marc Laird saw red and Millwall were left feeling blue as Orient took the derby honours courtesy of Wayne Gray's 50th-minute winner.
The O's had already edged ahead courtesy of Gray's close-range when Lions midfielder Laird was shown a straight red card following a challenge on John Melligan.
Even though they were a man, and a goal, down, Kenny Jackett's side were left rueing a succession of what at times were simply baffling decisions by the officials.
None more so than five minutes from time when Bas Savage was denied a blatant penalty when he was flattened two yards from goal as he was about to smack the ball home for what would have been a deserved Millwall equaliser.
But referee Andy D'Urso, who also missed Paul Robinson being wrestled to the ground by Brian Saah a split-second before, waved play on to the incredulity of the home fans and relief of the Orient bench.
Even O's boss Martin Ling sheepishly admitted afterwards: "We were lucky to get away with that one."
Not that his honesty will be of much comfort to The Lions, who were desperate to keep their winning run going following successive away victories at Swansea and Yeovil.
Unfortunately Den boss Jackett was dealt a triple blow before kick-off with strike duo Lewis Grabban (groin) and Gary Alexander (ankle), plus midfielder Alan Dunne (ankle) all ruled out having sustained injuries in Tuesday's win at Yeovil Town.
That meant a full debut for towering former Brighton frontman Savage, and a first start since 15th September for flying winger Ryan Smith.
On paper, The Lions starting line-up certainly looked blessed with pace, and it took just a couple of minutes before they were asking questions of the Orient defence.
Jay Simpson sprinted forward to charge down keeper Glenn Morris' clearance and the ball fell to Smith, who fired wide.
Millwall enjoyed the majority of possession in the opening half with Simpson, Smith and Danny Senda all whipping great balls into the box that flashed across the face of the goal.
Orient offered little as an attacking force and seemed content to defend in numbers, which they did stoically until the break.
Within five minutes of the second half Orient actually ventured forward, and nearly took the lead when Rhys Evans' clearance hit Gray, the ball spun up, dropped on the goal line and then swerved away.
However the relief in the Lions camp was short-lived and within seconds Gray stole in ahead of Senda to convert from close range with the home defence at sixes and sevens.
Matters went from bad to worse when Laird was dismissed for a two-footed challnge on Melligan.
It was going to be an uphill struggle for Millwall thereafter, but they gave it a go.
Simpson wriggled his way past two defenders and hit a fierce 20-yard drive that was parried by Morris at full stretch.
Neil Harris, a 74th-minute replacement for the hard-working Simpson, was denied at the far post after latching onto a long Senda through ball and Zak Whitbread was denied an 81st-minute equaliser when keeper Morris brilliantly tipped over the defender's goalbound header from a Smith corner.
But having had two penalty appeals turned down in the first half when Smith and then Senda were both sent sprawling, there quite simply was no arguing that a spot kick should have been awarded on 85 minutes.
A well worked freekick despatched by Ahmet Brkovic picked out sub Neil Harris, who had peeled away from his marker just inside the area. Harris headed back across goal, Robbo was pushed over, and as the ball fell to Savage, he too was fouled by Alton Thelwell as he prepared to shoot. Penalty? No chance.
A bad day at the office could have been even worse in the dying seconds when O's sub Andy Barcham lashed the ball wide with the goal begging.

Millwall: Evans, Senda, Frampton, Fuseini, Robinson, Whitbread, Laird, Simpson (Harris, 74mins), Savage, Martin (Brkovic, 55mins), Smith. Subs not used: Edwards, Bignot, Bakayogo.
Booking: Fuseini (44mins)
Sending off: Laird (57mins)

Leyton Orient: Morris, Thelwell, Saah, Melligan (Barcham, 75mins), Chambers, Gray, Thornton, Terry, Boyd (Ibehere, 72mins), Palmer, Demetriou. Subs not used: Nelson, Echanomi, Fortune.
Bookings: Saah (37mins), Demetriou (68mins), Thornton (74mins)
Scorer: Gray (50mins)

Ref: Andy D'Urso
Att: 10,986 (away 1,137)