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...
9. Liam Mandeville 7
Booked early on for bringing down Lolos to stop a counter and he fouled the same man seconds later which meant he had to watch his step. Put Chesterfield in front on 39 minutes when he collected a loose ball on the edge of the area and his curling strike took a deflection and flew in. Replaced by Dobra as part of a triple change on the hour. Photo: Brian Eyre
10. Dylan Duffy 7
Had Chesterfield's first attempt towards goal, curling narrowly wide early on, and put one on a plate for Dunkley with a cross which should have been converted. The wideman ended a lively half with a shot on target. Brought lots of energy to the side. Carried on where he left off after the restart, delivering an inviting cross for Bonis which should have been netted. Off for Berry just before the hour. Photo: Tina Jenner
11. Lee Bonis 6
Back in the line-up after his suspension in midweek. Had a good chance when he went through on goal from Braybrooke's excellent pass but he clipped wide as the angle narrowed. And he should have scored early in the second 45 from Duffy's cross but his close-range header was soft. His runs in behind caused problems and he led the line well. Replaced by Grigg on 60. Photo: Brian Eyre
12. James Berry 6
On for Duffy on the hour. One cross was begging to be finished but nobody was on the end of it. Slipped in Dobra with 10 to go as well. Switched to the right when Markanday went off. Photo: Tina Jenner