Barnsley were badly depleted by injuries and suspensions, and were forced to pair full-backs Bobby Hassell and Tom Williams at the heart of their defence following the late withdrawal of skipper Paul Reid with a foot injury.
In an open game it was the visitors who provided the better football but Barnsley enjoyed the more clear-cut chances.
They twice had the ball in the net in the opening half hour but both were ruled out for offside.
The first effort from on loan striker Daniel Nardiello was very tight and clearly the Barnsley bench thought the official had got it wrong.
Fellow on loan forward Simon Johnson twice tested keeper Matt Baker with decent efforts, while at the other end the flame haired Dean Lewington and midfielder Gareth Edds tested out Barnsley's teenage goalkeeper Scott Flinders who was making his home debut.
The best chance in the opening 35 minutes fell to Nardiello but he side-footed wide when sent clear by Johnson.
But with virtually the last kick of the half the home side got their noses in front. A brilliant lofted pass from former Republic of Ireland international Stephen McPhail sent Nardiello clear and he finished clinically to Baker's left.
Barnsley enjoyed the better of the second half but were stunned by a 55th minute equaliser.
Lewington's left-wing corner wasn't cleared by the Barnsley defence and centre back Michel Pensee-Bilong finished with a left-foot volley from six yards.