Barnsley slipped out of the League One play-off places after a 1-1 home draw with high-flying Preston North End on Saturday.
Jabo Ibehre got his first Barnsley goal to equalise late on after Joe Garner had given the visitors an early lead. The Reds have now slipped down to seventh with Peterborough overtaking them.
Josh Scowen and Ben Pearson both missed the game through injury and illness respectively and were replaced by James Bailey and winger Cameron Stewart, who signed the morning of the game on loan from Ipswich.
After using a 4-4-2 in the goalless draw at Leyton Orient on Tuesday, Reds head coach Lee Johnson reverted to a 4-5-1 with Sam Winnall on the left, Stewart on the right and John O'Sullivan, Conor Hourihane and Bailey in the middle.
Seconds after O'Sullivan hit the post with a long-range shot, Reds goalkeeper Adam Davies kept up his sensational form when he denied Garner from two yards out.
Preston's Tom Clarke blazed over inside the box and Callum Robinson brought another good save out of Davies from a similar position.
The deadlock was eventually broken on 37 minutes when Robinson beat Mason Holgate and swung in a cross from the left which fell to Garner who fired into the bottom right corner from eight yards out.
Stewart should have levelled early in the second half but blazed over the bar eight yards out as the goal was gaping following a Declan John cross.
Ibehre levelled on 79 minutes when he headed in Holgate's excellent right wing cross from close range.
