Barnsley’s League One play-off hopes are all but over after they crashed to defeat at relegation-threatened Colchester.
Prospects looked rosy for the Reds at half-time thanks to George Waring’s 40th-minute opener.
However, Lee Johnson’s men collapsed to a second straight defeat after the restart thanks to strikes from Gavin Massey, Jacob Murphy and George Moncur.
With just five games left to play, the Reds are now seven points adrift of sixth-placed Chesterfield, though they do welcome the Spireites to Oakwell on Saturday.
Ben Pearson came close to doubling Barnsley’s lead shortly after the restart, but once Massey had equalised, the U’s took control in the closing stages.
Waring scored five minutes before the break to hand Barnsley the lead in their first-ever visit to the Weston Homes Community Stadium.
Waring - on loan from Stoke City - notched his sixth Reds goal in scruffy fashion, bundling home Conor Hourihane’s free-kick from close range as he just beat Lewin Nyatanga to the ball.
But Colchester equalised after 62 minutes through Gavin Massey, and then completed a dramatic turnaround when Jacob Murphy headed into an open net to put his side 2-1 up.
Reds sub Sam Winnall then had a goal disallowed after he had rounded Sam Walker and slotted home.
And Colchester then sealed the points with four minutes remaining when Massey set up George Moncur to finish.
Earlier in the first half, there had been few clear-cut chances.
Barnsley’s best came from Waring, who saw a goal-bound header deflected inches wide.
Hourihane had also seen a smart shot blocked by South African defender Bongali Khumalo.
Barnsley boss Lee Johnson made four changes to the team beaten for the first time in 10 games on Saturday for today’s crunch clash at relegation-threatened Colchester.
In the wake of the derby defeat by the Blades at Oakwell at the weekend, Johnson drafted James Bailey and Milan Lalkovic back into the midfield, while Waring replaced Winnall up front.
Adam Davies was back between the sticks following his recent injury, replacing loan man Joe Wildsmith.
Meanwhile, loan front-man Jabo Ibehre was unable to feature against his parent club.
The U’s - currently second-bottom in League One - have boosted their survival hopes recently by pocketing seven points from their last three games.
Their boss Tony Humes made one change to the line-up which won at Port Vale on Friday, with Sammie Szmodics stepping back into the midfield at Alex Gilbey’s expense.
The Reds started the game brightly, and Hourihane saw his shot blocked before Lalkovic was flagged offside.
Mason Holgate then almost caught out Colchester goalkeeper Walker with a swirling cross as Barnsley continued to dominate proceedings.
And they went ahead in the 40th minute through Waring, as he touched home Hourihane’s cross to give his side the lead.
Colchester (4-3-3): Walker; Brindley, Khumalo, Eastman, Wynter; Moncur, Szmodics, Lapslie; Porter, Massey, Murphy.
Subs: Lewington, Vincent-Young, Harney, Gilbey, Healey, Bonne, Sembie-Ferris.
Barnsley (4-5-1): Davies; Holgate, Cranie, Nyatanga, John; Scowen, Bailey, Pearson, Hourihane, Lalkovic; Waring.
Subs: Dibble, Berry, Winnall, Smith, Ramage, Stewart, Hemmings.
Referee: Keith Hill (Hertfordshire).