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...
5. Tom Pearce 7
The left-back had a dangerous drive towards goal blocked in a hectic period of end-to-end action. He later made an important block down the other end from Gordon. Showed what a wand of a left-foot he has when his whipped free-kick was nodded wide by Grigg. Photo: Tina Jenner
6. Ryan Stirk 6
Battled away on a bobbly pitch. A scrappy, hard-fought performance. Put his body in the way of things and pressed. Gritty outing. Photo: Tina Jenner
7. Sammy Braybrooke 7
Put Bonis through on goal with a beautiful defence-splitting pass and he did the same for Mandeville midway through the first-half. Steady after the break. Photo: Tina Jenner
8. Dilan Markanday 6
Had a huge chance when he was slid in on goal by Bonis but he dragged wide. Wasn't pretty but he worked hard for the team. Off for Naylor with 15 to go. Photo: Tina Jenner