Lee Bonis, Chey Dunkley and Dilan Markanday all missed big chances for the Spireites before Liam Mandeville’s deflected strike put them ahead on 39 minutes.
Bonis squandered another good opportunity after the break, while Ronan Darcy struck the posts for the hosts, before Armando Dobra was denied by goalkeeper Jacob Chapman in the closing stages.
The Blues were on track for successive wins and clean sheets and those missed chances came back to bite them when Jay Williams headed in with the last touch of the game in the 95th minute.
The result means Chesterfield drop a place to seventh despite being unbeaten in six.
Here are our player ratings from the game...
13. Will Grigg 6
Replaced Bonis on 60 minutes. Glanced a free header from Pearce's superb cross wide which left the striker with his head in his hands. Came close to turning in a Dobra low cross, too. Photo: Brian Eyre
14. Armando Dobra 6
Came on for Mandeville for the last half an hour. Slipped in Berry late on but his outside-of-the-boot finish was parried away. Photo: Brian Eyre
