Ten-man Barnsley blew a great chance to move within two points of the Championship play-off places as they were humbled by rampant Nottingham Forest.
The Reds led twice during what proved to be a frantic opening 45 minutes, but disappointing defending helped the visitors roar back as they secured a second straight win on the road.
Henri Lansbury bagged a superb hat-trick, the first of his career in what was his 261st senior appearance.
The hosts’ four-match unbeaten run was ended emphatically on a night when goalscorer Marley Watkins was also red-carded in the second period.
Boss Paul Heckingbottom made just the one change to his starting line-up in the wake of last weekend’s goalless home stalemate against struggling Wigan.
It was a significant alteration, though, with Adam Hammill finally ending his month-long exile from the XI as he was recalled in favour of Liverpool loanee Ryan Kent.
As far as under-pressure Forest boss Philippe Montanier was concerned, he still made a handful of changes despite last weekend’s first away victory of the season at Ipswich.
They were missing injured front-man Britt Assombalonga, who scored both goals in a 2-0 win at Portman Road.
As it turned out, Assombalonga certainly wasn’t missed during what turned out to be a first-half-goal-fest.
It all started after just five minutes, with Sam Winnall diving in bravely to head home James Bree’s cross as last season’s top scorer registered his fifth goal of this term.
Eight minutes later Forest levelled, with Lansbury drilling home low into the corner of the hosts’ net from a free-kick some 25 yards out.
While some travelling fans were still celebrating, the Reds roared back into the lead just 73 seconds later.
This time Watkins notched his fifth of the season, springing the offside trap as he met Conor Hourihane’s cross before smashing past ‘keeper Vladimir Stojkovic.
At that point the Reds were cruising, but by the interval they had fallen behind.
Greek striker Apostolos Vellios - Assombalonga’s replacement - evened it up again after 24 minutes, this time scooping a close-range effort over Adam Davies after meeting Daniel Fox’s cross.
Winnall and Hammill (twice) came close to a third Reds goal, but Forest struck again in the second minute of added time.
Lansbury made it 3-2, heading home a smart cross from Ben Osborn.
It had been a breathless first period at Oakwell.
The hosts made a terrific start to the second half.
In search of an equaliser for the first time, Winnall saw a header saved as he met a decent cross from youngster Bree, who was making his 50th appearance in a Barnsley shirt.
Midfielder Josh Scowen also tried his luck from 20 yards, only to see a shot just clear the crossbar.
It was against the run of play when Forest made it 4-2 in the 63rd minute.
Osborn was the scorer this time, leaping well to head home Michael Mancienne’s precise cross.
As if that wasn’t bad enough, the Reds then gave themselves an absolute mountain to climb when Watkins was shown a straight red card two minutes later, after appearing to stamp on Dutchman Thomas Lam.
Lansbury made it 5-2 from the penalty spot with eight minutes to go after Scowen had clumsily bundled over sub Matt Cash just inside the box.
Barnsley (4-4-2): Davies; Bree (Janko, 74), Roberts, Jackson, Yiadom; Watkins, Scowen, Hourihane, Hammill; Winnall (Williams, 88), Armstrong (Bradshaw, 64). Subs not used: Townsend, MacDonald, Morsy, Kent.
Nottingham Forest (3-5-1-1): Stojkovic; Mills, Perquis, Worrall; Mancienne (Vaughan, 86), Kasami, Lansbury, Lam, Fox (Cash, 78); Osborn; Vellios (Bendtner, 70). Subs not used: Henderson, Carayol, Lica, Grant.
Referee: Oliver Langford (West Midlands).