A frenetic three minutes of injury time just about livened up this relegation dogfight.
Brentford struggled to create any chances in the first half. Two long-range strikes both from free kicks by Stephen Hunt and Michael Dobson and an Eddy Hutchinson header were all the Bees could manage.
Chesterfield were by far the better team in the opening period as Brentford struggled to defend against set pieces.
Ian Evatt saw Bees keeper Alan Julian parry his far post header from a corner. A poor back-header by Ronnie Bull failed to go unpunished by Chris Brandon as he could only fire straight into Julian from six yards.
Evatt thought he had given Chesterfield the lead on the stroke of half time as he headed past Julian from yet another corner, however the goal was disallowed for pushing by referee Tomlin.
The second half was all about Brentford as they pushed for the three points. The home side never looked like scoring despite all their pressure.
Steve Blatherwick and Evatt were outstanding in the Spireites defence, denying the Bees any clear-cut chances. It took until the second minute of injury time to break the deadlock.
Brentford substitute Steve Evans looked offside but there was no flag and he fired a left foot shot past Carl Muggleton.
Brentford were denied a victory with the last touch of the game as Evatt powered his header from Brandon's free kick past a despairing Julian.
"It would have been a total injustice had we lost that game today," said Chesterfield boss Roy McFarland.
"We were reasonably comfortable and all we had to do was not let them score, but give us credit we came back and got the equaliser." Martin Allen praised his Brentford team for their second half performance. "When it's not flowing it's horrible sitting in there at half time knowing that you've got to go back out," Allen said.
"We stepped on the gas in the second half and we kept going right until the end and that's all credit to the players."