George Friend's 97th minute equaliser for Middlesbrough dashed Millwall's hopes of gaining an opening-day victory at The Den as the two clubs fought out a 2-2 draw.
As the stadium celebrated its 25th anniversary, the attacking duo of Aiden O'Brien and Lee Gregory fired home to put The Lions 2-0 up at the break. Martin Braithwaite ensured a nervy final period with a consolation a few minutes from time before Friend poked home the equaliser seven minutes into five signalled.
Lions boss Neil Harris decided to start the season with a familiar looking line-up, complete with Jake Cooper and Shaun Hutchinson at the heart of the defence, Jed Wallace and O'Brien on the wings and Steve Morison up front alongside Gregory.
Millwall started the match - and season - by pressuring their hosts, but the visiting back line stood firm in the face of a couple of in-swinging crosses. Boro's first shot, meanwhile, came from Braithwaite - the attacker giving Jordan Archer an easy save following a low grubber.
On 12 minutes, Millwall took the lead through O'Brien. A superb piece of play between Morison and Gregory set up the latter to fire a low cross into the box where the Irishman, fresh from signing a new long-term contract, was waiting to slam home the opening goal of the campaign.
The Lions continued to probe following the opener and could have had a second in the 18th minute, but O'Brien was just unable to connect with Morison's cross. From the resulting Shaun Williams corner, Cooper headed just wide.
On 37 minutes, some outrageously kamikaze defending from Tony Pulis' side gave Millwall a second goal of the afternoon. A questionable backpass set Gregory into a foot-race with Darren Randolph and after the striker's first bite of the cherry was just about stopped by the 'keeper, the second effort was a simple roll into the net from yards out.
Seven minutes into the second-half it was nearly three, but Morison agonisingly could not reach Gregory's teasing cross at the back post.
From there The Lions had to be on their guard defensively - Hutchinson bravely threw himself in front of Britt Assombalonga's header just before the hour-mark - but were also sharp in attack. Gregory's volley from a Morison cross was blocked before O'Brien narrowly missed Wallace's vicious ball into the box.
More heroic defending was needed on 63 minutes as Millwall's defence were on hand to block what looked a certain goal from Assombalonga, before both Hutchinson - who suffered a cut to the head in the first-half - and O'Brien needed attention from the physio after defending a corner-kick.
Just over 15 minutes from time Ryan Shotton had Archer scrambling as he headed a free-kick just past the post, with the same combination forcing a corner-kick seconds later as the stopper had to tip a cross over the bar. Aden Flint then got his attempt wrong on the edge of the six-yard box as the visitors looked to halve the deficit.
With 83 minutes on the clock a the trio of Mahlon Romeo, Wallace and Morison created an opening for the latter, but his shot was deflected behind. Jiri Skalak stepped off the bench with five minutes remaining, replacing goalscorer O'Brien, but slack defensive play allowed Braithwaite to ghost in and poke home Boro's first goal with three minutes to go.
That prompted Harris to bring on Murray Wallace for his debut and a switch to a five-man defence, with the inevitable late pressure following. Substitute Marcus Tavernier blazed over into five minutes of injury-time before the dagger into Millwall hearts came via Friend as the clock struck 97 minutes.
Millwall: Archer, Meredith, Hutchinson, Cooper, Williams, J. Wallace, Gregory (Elliott 80'), Romeo, Morison (M. Wallace 89'), O'Brien (Skalak 85'), Saville. Subs not used: Amos, McLaughlin, Onyedinma, Tunnicliffe.
Scorer: O'Brien 12', Gregory 37'
Middlesbrough: Randolph, Friend, Shotton, Leadbitter (Wing 55'), Clayton (Tavernier 59'), Assombalonga, Braithwaite, Howson, Downing (Fletcher 74'), Fry, Flint. Subs not used: Dimi, Johnson, Wood, Chapman.
Scorer: Braithwaite 87', Friend 90+7
Booking: Clayton, Wing
Referee: Kevin Friend
Attendance: 15,056 (1,768)