Promotion-chasing Wanderers were staring at only a second home league defeat of the season after veteran hit-man Jack Lester had given plucky Chesterfield a 31st-minute lead.
But the former Grimsby striker turned villain moments after the restart for the second half when he saw his penalty saved by Jamie Young.
And he was made to pay at the death when his defenders failed to clear a Nathan Ashton free-kick, swung in from the left. It fell to Pittman, who swept home his third goal of the campaign from 12 yards out. Ashton had only come on as an 87th-minute substitute!
It was tough luck on mid-table Chesterfield, who will be rueing two points dropped after seeing Lester miss from the spot.
Chris Zebroski could have equalised for the Chairboys right on half-time, but saw his looping header cannon back off the bar.
After the penalty miss Wycombe, who had been slow out of the blocks in the first half, were blatantly boosted.
But Lester even had two more chances to double his side's lead, seeing a header deflected over from a corner and a shot saved again by Young from a mad scramble in the six-yard box.
Matt Harrold seemed to get the flow of the game going in the right direction for Wycombe with some good hold-up work in the last 15 minutes. And the hosts were finally rewarded thanks to Pittman's last-gasp leveller.
They are now four points behind Brentford at the top, although Peter Taylor's men have a game in hand.