Edward Upson scored a late winner to hand Championship newcomers Yeovil Town three points against Millwall.
Danny Seaborne went close with a first-half header for the visitors at the Den before Marek Stech pulled off a superb save to deny Millwall's James Henry.
The Lions midfielder then shot over and wide in front of an empty goal in the second half after a mistake by Stech.
And Upson punished them with a powerful strike to secure the victory for Yeovil with two minutes remaining.
It was a big day for both sides at the Den as Yeovil featured in the second tier for the first time in their history and Steve Lomas took charge of his first game as Millwall boss after replacing Kenny Jackett in the summer.
The former St Johnstone manager watched on as his side got off to a good start with Shane Lowry crossing for Steve Morison early on, but the on-loan Leeds striker could not keep his free header on target.
Debutant defender Seaborne then went close for the visitors but Millwall keeper David Forde managed to save his header on the line.
Yeovil goalkeeper Marek Stech kept the game level when he smothered the ball to deny Millwall's Henry who looked certain to score after a one-two with Morison.
Henry continued to cause problems after the break, but when Morison capitalised on confusion in the Yeovil defence and the ball sprung to the midfielder he fired off target with Stech stranded.
Millwall were punished for their failures in front of goal in the final stages when they failed to deal with a corner properly and the ball fell to Upson, who found the bottom corner from the edge of the area.
Millwall manager Steve Lomas:
"The defeat is very hard to take but we've only got ourselves to blame. We had five decent chances and you have to take them.
Yeovil Town were promoted to England's second tier last season after victory in the League One play-offs after 10 seasons in the Football League
"I can't fault the lads for their competitiveness and will-to-win, but you have to show that little bit of know-how.
"Anybody can go and smash into tackles, but it's about wanting to get on the ball, having the hunger and desire to show what you can do."
Yeovil Town manager Gary Johnson:
"We are used to people telling us we are going to get relegated and with our budget we really should be, but we use it as a strength.
"Anything we do is a bonus. But we have a group of lads who know how to put in a shift and work for each other. They will all improve as they are a young lot.
"I think we deserved that win. I thought we played the best side in the first half without having any real chances."