Sunderland secured their place in English football's Championship play-offs with a 3-0 win away to Preston on Monday but only after Millwall suffered an extraordinary loss at home to Blackburn Rovers.
The final day of the regular second-tier season kicked off with Millwall knowing a win would guarantee their place in the play-offs that decide the last available spot on offer in next term's lucrative Premier League.
Burnley, the champions, and second-placed Sheffield United were already assured of automatic promotion.
But the identity of the clubs finishing fifth and sixth -- the teams that would join third-placed Luton and fourth-placed Middlesbrough in the play-offs -- had still to be decided.
All seemed set fair for south London club Millwall when they led 3-1 at the interval following two goals from Duncan Watmore and one from Oliver Burke.
But with Joe Rankin-Costello having reduced the deficit, Ben Brereton Diaz's second-half double inspired a Rovers revival as they won 4-3 at the New Den.
"As soon as their (Blackburn's) second (goal) went in, you could see the nerves appear in the team," said Millwall manager Gary Rowett.
"We got ourselves into a brilliant position but you have to handle these big moments, you have to do the right things and show control and calmness. That was something we needed to do better."
He added: "In some ways, I'd have taken losing 1-0 more easily than losing 4-3 from 3-1 up as it feels like we've let it slip through our fingers."
Sunderland's win at Preston meant the northeast side took sixth place, with Blackburn leapfrogging Millwall only to be denied a play-off position of their own on goal difference.
"I felt as though we could come here and control the game," said Sunderland manager Tony Mowbray.
He added: "We didn't have harsh words at half-time, but we discussed that we needed to control the game better. We became more confident and, off the back of the first goal, we controlled things. It was a solid performance."
Coventry, meanwhile, claimed a final day point to finish fifth and secure a play-off berth following a 1-1 draw at Middlesbrough -- who they will now meet in a two-legged semi-final.
Gustavo Hamer put the Sky Blues ahead before Cameron Archer equalised in first-half stoppage time.
West Brom, the other team that started the day with a chance to finish in the top six suffered a 3-2 defeat at Swansea, for whom Joel Piroe scored a stoppage-time winner.
Luton drew 0-0 at home to Hull and will now face Sunderland in the play-off semi-finals.
Burnley won 3-0 at home to Cardiff, with Josh Brownhill, Ashley Barnes and Scott Twine all scoring at Turf Moor as Vincent Kompany's team finished with 101 points, while Sheffield United won 2-1 at Birmingham.
Of the three relegated sides, Blackpool won 1-0 at Norwich thanks to Morgan Rogers's early goal and basement club Wigan drew 0-0 at home to Rotherham.
Meanwhile, Reading lost 2-0 at Huddersfield, who had already secured their Championship status with a game to spare.
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