Barnsley lost the leadership of the second division after conceding a 90th minute goal to Notts County super sub Paul Heffernan.
The Yorkshire side had stormed to the top of the table on the back of three wins and a draw, and were overwhelming favourites to see off a Magpies side that was rooted to the bottom of the table, pointless and goalless.
But County broke their duck in dramatic style as Heffernan fired past Sasa Ilic in the last minute.
Barnsley had been the better side in the first half, although not as impressive as they had been when they won 2-0 at Blackpool a week earlier.
Kevin Betsy, who had grabbed the only goal to help Barnsley to victory in their previous home game against Brighton, missed two excellent chances this time around.
Barnsley boss Gudjon Thordarson was not too down-hearted to have let the game slip saying: "I wanted three points and I was disappointed to concede such a late goal.
"But it has still been a very encouraging start to the season. We remain unbeaten and are well-placed in the table." Barnsley had made the breakthrough two minutes after half time when a hopeful looking ball from Lee Crooks was misjudged by Notts County defender Tony Barras and Dutchman Dean Gorre pounced to slot home his fourth goal of the season.
That should have given Barnsley the green light to go on and secure a comfortable win but although Anthony Kay was unlucky with a fierce drive that was well-saved by Steve Mildenhall, Barnsley on the whole sat back and looked content to hold out for a 1-0 win.
That policy backfired as Heffernan struck in the 90th minute.
There were chances at both ends in injury time, Clive Platt heading wide from just six yards for County and then Betsy fluffing an equally good chance when set up by the unselfish Gorre.