Andy Yiadom struck an extra-time winner as plucky League Two newcomers Barnet dumped Millwall out of the Capital One Cup with a shock 2-1 win at The Den.
Barnet took the lead after 11 minutes with a John Akinde penalty, and although Steve Morison rattled the woodwork with a 61st-minute spot-kick, The Lions striker levelled with a fine header on 76 minutes to send the game into extra-time.
Buoyed by Saturday's opening day win at Shrewsbury, Millwall had high hopes of booking their place in Thursday's Second Round draw, but they came unstuck against a determined Bees outfit.
Manager Neil Harris made four changes to his starting XI, handing full debuts to goalkeeper Jordan Archer and left-back Shane Ferguson, with John Marquis and Sid Nelson also coming into the side.
However Archer's first real action of the night was not what he envisaged, picking the ball out of the back of the net following Barnet's opening goal.
Akinde used his power and pace to burst past Byron Webster on the left flank, but the Lions defender seemed to have recovered sufficiently to put the Barnet player off his stride as they headed into the penalty area. However Akinde went to ground and to the disbelief of Webster, and howls of protests from home fans in the Cold Blow Lane Stand, referee Andrew Madley pointed to the spot.
Akinde stepped up and promptly sent Archer the wrong way, burying his penalty into the bottom right corner.
Millwall responded and twice created half-chances when Webster sent a looping header from a Ferguson free-kick narrowly over on 21 minutes before Lee Martin curled a 25-yard free-kick off target three minutes later.
However, The Lions were indebted to Archer for keeping the busy Bees at bay, getting a strong hand to divert Luke Gambin's low goal-bound effort round the post for a corner.
Millwall had a great opportunity to level on 32 minutes; Morison showed real persistence to win the ball 30 yards from the Barnet goal and then threaded an inch perfect ball through to Martin in a one-on-one with Graham Stack. The Bees keeper spread himself expertly to block Martin's shot and although the loose ball seemed to be spinning over the line, a last-ditch clearance from Sam Togwell diverted the danger inches from the goal.
Chopper's men started the second-half brightly and were unlucky not to level when skipper Tony Craig's blockbuster of a volley was blocked seconds after the restart.
But it took another piece of athletic goalkeeping from Archer to prevent the visitors doubling their tally just before the hour mark when Michael Nelson caught a volley flush from 12 yards, but was thwarted by the stopper at full stretch.
The Lions were handed a great chance to level with a penalty of their own, awarded after the dangerous Fred Onyedinma burst into the danger zone and was caught by Gambin, who was penalised for a trip.
Morison, making his first appearance at The Den after signing for a third spell with the club, stepped up but smashed his spot-kick against the crossbar and the ball bounced the safety.
Having introduced fresh legs in the shape of Paris Cowan-Hall and Lee Gregory, the home side pressed again. Cowan-Hall sprinted down the right and picked out the onrushing Onyedinma at the back post, but the latter connected with the ball slightly off balance and could only drill his effort wide of the target.
With 14 minutes remaining Millwall's persistence paid off as Morison met an inviting free-kick delivery from Shaun Williams and sent a looping header past former Lions keeper Stack for the equaliser.
The two teams traded scoring chances with Archer acrobatically keeping out a fierce shot from Gavin Hoyte and then both Morison and Webster going close for the home side, first with a shot across the face of goal and then a header cleared off the line by Bira Dembele.
Cue extra-time and that man Akinde, a thorn in Millwall's side all night, turned from goal scorer to provider in the 102nd-minute. Akinde picked up the ball on the right, drove down the line and despatched an inviting cross that Yiadom swept home with aplomb.
Despite a little flurry of pressure in which Ed Upson twice had shots saved by Stack and sub Mark Beevers hit a stinging shot that the keeper did well to hold, Millwall just could not find a way through a well organised Bees backline as the League Two team booked their place in the hat for Round Two.
Millwall: Archer, Nelson, Ferguson, Craig, Webster, Onyedinma (Beevers, 90+5mins), Upson, Williams, Marquis (Gregory, 66mins), Morison, Martin (Cowan-Hall, 66mins). Subs not used: Forde, Cummings, Powell, Philpot.
Booking: Martin (53mins)
Scorer: Morison (74mins)
Barnet: Stack, Hoyte, Johnson, Dembele, Nelson (N'Gala, 89mins), Yiadom, Weston (Gash, 69mins), Akinde, Gambin (Vilhete, 69mins), Togwell, Champion. Subs not used: Stephens, Tomlinson, Muggleton.
Booking: Champion (81mins)
Scorers: Akinde (11mins, pen), Yiadom (102mins)
Ref: A Madley
Att: 4,454 (away 527)