The Spireites fell to a 4-1 Walsall defeat after a pivoital second-half minute where the Saddlers took the lead and Dion Donohue saw red in the aftermath.
After a dominant start from the visitors the Spireites took the lead against the run of play, and had a huge helping hand when Walsall defender James O'Connor flicked a header past his own 'keeper from Jay O'Shea's cross.
Spireites centre-back Tom Anderson then scored another own goal to level matters before the break as he swung a boot and could only shin into his own net to bring Walsall level.
The game took a major swing on the hour mark as Sam Mantom stroked home to give the visitors the lead and in the resulting melee Donohue received a straight red card.
It got worse when Jordy Hiwula pounced on an Anderson mistake and made it three and fourth soon followed when Mantom was played into space and finished cooly for his second.
Walsall made the early running with Tom Bradshaw heading wide from Mantom's cross and Rico Henry seeing his low, driven shot deflected wide.
As the visitors continued to press, O'Connor had an effort deflected wide and Mantom missed the target with a low shot.
The Spireites then enjoyed a good spell which resulted in a goal. O'Shea fired over from inside the area and then Lee Novak got on the end of Ollie Banks' chip into the area, forcing 'keeper Neil Etheridge to get down bravely to smother his shot.
The breakthrough came in the 18th minute when O'Shea floated the ball into the area and O'Connor, who was attempting to clear, succeeded only in diverting the ball into his own net.
As the Spireites looked to build on their lead, Donohue got to the left-hand byline and delivered a cross which was too high for Lee Novak.
Connor Dimaio then tried a shot from distance which was straight at Etheridge.
Tommy Lee dealt comfortably with a shot on the turn from Hiwula.
Drew Talbot was the next player to try his luck from long range and his effort was well off-target.
Walsall drew level in the 35th minute when Anderson attempted to cut out Mantom's low cross from the right and sliced his attempted clearance with the ball going past Lee and into the net.
Banks went close soon after with a shot from distance which was only a fraction off-target.
O'Shea failed to make the most of a good opening when the ball came quickly to him just inside the area.
The final moment of the half came when Jason Demetriou tried a curling shot which went straight to Lee.
Early after the re-start, O'Shea attempted a shot from distance which was not far off-target.
Walsall took the lead when Demetriou played the ball back to Mantom (57) who beat Lee with a low shot from the edge of the area.
Chesterfield were reduced to ten men two minutes later when Donohue was shown the red card after being involved in a scuffle in the aftermath of the goal.
Henry had a great chance to put the visitors further in front, when his close-range header was saved by Lee.
Shortly afterwards, Walsall did score when a mistake from the unfortunate Anderson allowed Hiwula (71) to run through on goal and he slotted past Lee who got a hand to the ball but couldn't keep it out.
Mantom (75) scored his second of the afternoon, stroking the ball past the exposed Lee after some neat passing around the edge of the area.
Substitute Ritchie Humphreys had a good chance to pull a goal back when the ball fell to him in a good position inside the area, but he shot wide.
Chesterfield: Lee (gk) - Donohue, Hird, Anderson, Talbot (John 68') - O'Shea (Ebanks-Blake 74'), Banks, Liddle (c), Dimaio, Ariyibi - Novak (Humphreys 79')
Subs not used: Chapman (gk), Simons, Raglan, Slew
Walsall: Etheridge (gk), Demetriou, Taylor (Forde 79'), O'Connor, Downing, Chambers, Mantom (Morris 88'), Bradshaw (Lalkovic 84'), Sawyers, Henry, Hiwula
Subs not used: MacGillivray (gk), Preston, Flanagan, Kinsella
Chesterfield: O'Connor (og) 19'
Walsall: Anderson (og) 35', Mantom 57', 75', Hiwula 71'
Attendance: 6,938 (823 away fans)