High-flying Chesterfield were caught out by a smash-and-grab raid from lowly Barnet.
The Spireites, with four straight wins behind them, struggled to find their form in a frustrating display.
Striker Jack Lester, looking to score for the seventh consecutive game, was well-marked by Sagi Burton and had few chances in a scrappy game.
Chesterfield were regularly caught by the Barnet offside trap and were eventually beaten by a breakaway attack.
The Bees showed some signs of threat in the opening 45 minutes but failed to trouble home keeper Barry Roche.Kenny Gillet shot wide from 25 yards while Jason Puncheon had an effort charged down by a defender.
Chesterfield should have gone in front on 30 minutes when a shot from Lester was pushed out by Barnet keeper Lee Harrison. The ball fell to Jamie Lowry but from six yards out he failed to find the net.
Chesterfield found it difficult to get possession and eventually fell behind in the 69th minute from Barnet's first shot on goal.
Substitute Nicky Nicolau sent over a cross from the left which was headed on by Adam Birchall. Roche made the save but could only direct the ball into the path of Jason Norville who headed in from close range.
The goal sparked some life into the home team and Lester went close to grabbing an equaliser three minutes later. His low shot beat Harrison but Nicolau was back to clear off the line.
Chesterfield went close again in the 83rd minute when a corner from Peter Leven found the head of skipper Aaron Downes. The centre-back got plenty of power behind his effort but was denied by an excellent save from Harrison.
Chesterfield almost conceded a second goal in the 89th minute. Birchall was left unmarked inside the box but shot straight at Roche whose save spared the home team even more embarrassment.