Barnsley were brought back down to earth with a bump following their Wembley win last weekend.
Goals from Lee Novak and Ollie Banks either side of an Adam Hammill strike ensured their play-off hopes were dented at Oakwell by Danny Wilson's Chesterfield.
James Bree came in to replace George Williams at right-back in the only change.
The game also marked the first return of Wilson since his dismissal last year.
Wilson named ex-Red Jamal Campbell-Ryce in the starting team alongside Barnsley-born Drew Talbot and Ian Banks' son Ollie Banks.
The visitors edged a quiet opening with Gboly Ariyibi going close.
They should have really gone ahead when Ariyibi's cross-cum-shot was miraculously turned away from goal on the line by teammate Lee Novak.
Adam Davies then pushed over Charlie Raglan's header from Jay O'Shea's cross.
Sam Winnall had Barnsley's best chance of the first half but his shot from Bree's cross was straight at Chesterfield 'keeper Tommy Lee.
Lloyd Isgrove then failed to get a crucial touch on to Hammill's in-swinging cross.
Ariyibi's cross found Lee Novak for the opening goal on 57 minutes. Novak drifted to the far post and guided a header beyond Davies into the bottom corner.
Barnsley responded well with Isgrove firing a deflected shot off the post and Winnall's header had to be clawed out by Lee.
Marley Watkins and Ivan Toney were brought on after the hour mark for Isgrove and Ashley Fletcher.
Winnall again tested Lee but the 'keeper pushed away his curling effort.
Toney, who had earlier gone down in the box but was denied a penalty, then fired straight at Lee after being slipped in by Josh Brownhill.
Hammill then produced another surging run and finish from midfield to level on 75 minutes. His shot took a deflection on its way to beating Lee.
Chesterfield had to see out the final ten minutes with ten men after Campbell-Ryce was handed a second yellow for fouling White.
Barnsley were then sucker-punched. Ariyibi wriggled free down the left and put a cross on a plate for Banks to smash in.
The Reds came agonisingly close to a leveller in the closing stages.
Hammill's shot was cleared off the line and from the next passage of play Marc Roberts' header came off the post and Brownhill couldn't get enough contact on the rebound.
