Chesterfield took the lead twice but were pegged back on both occasions by Leyton Orient’s David Mooney before Dean Cox took full advantage of an injury time mistake by Tommy Lee.
An Elliot Omozusi own goal had restored the Spireites’ lead ten minutes in to the second half, after Mooney had cancelled out Jay O’Shea’s penalty kick during a flat first 45 minutes.
Lee had made several crucial stops during the match but an uncharacteristic error in the 11th minute of time added on handed the League One strugglers three valuable points.
The result meant Chesterfield remain outside of the play-off picture ahead of Tuesday night’s trip to Scunthorpe United while the visitors Orient jump off the bottom of the table.
Chesterfield boss Paul Cook made three changes to the starting line up to that which lost 2-0 at home to Preston in the week. Charlie Raglan, who played more than 85 minutes on Tuesday, replaced the suspended Sam Hird who served his suspension.
Dan Jones and Byron Harrison dropped to the bench. Armand Gnanduillet and Mark Duffy, who came in to the side to make his first start, were selected as Cook opted to maintain a 4-4-2 formation that saw loanee Caolan Lavery selected in attack.
It meant Sam Clucas filled in at left back but the former Mansfield Town man showed his attacking intent when he reached the byline in the Spireites first meaningful foray forward. Shane Lowry, however, was well placed to clear the cross.
The visitors looked organised but let themselves down in the final third more than once. Bradley Pritchard over hit a cross when put through by an incisive Romain Vincelot pass then Omozusi did the same when trying to find Pritchard on the over lap.
Chesterfield started to go through the gears and began to pass the ball around quickly on the edge of Orient’s penalty area, Duffy saw a shot blocked by Vincelot’s sliding challenge before the breakthrough came after some more neat foot work.
Lavery was slid in the inside left channel, squared Nathan Clarke up and knocked the ball past him to work the shooting opportunity only for the Orient skipper to trip the young striker from Sheffield Wednesday up. O’Shea sent Alex Cisak the wrong way from the spot.
The visitors looked to respond immediately and Cox forced Lee in to a save diving low to his right and conceded the corner. O’Shea was alive at the other end to try a curler in to the top corner that was straight at Cisak and the keeper claimed.
Lee had to be on his toes moments later when Chris Dagnall was sent in the clear but the Chesterfield keeper was first to the ball and when it kindly fell for Mooney on the volley, the Orient striker scuffed a left-footed effort harmlessly wide of the unguarded net.
It was a little more straight forward for Lee to prevent Cox’s strike as the visitors pressed for an equaliser. Dagnall was again put through after Omozusi cut out a Spireites breakaway, controlled well but completely missed his shot under pressure from Ian Evatt.
Chesterfield were warned and the leveller soon came when Cox broke through but, rather than taking on Lee, squared the ball for Mooney to tap in from two yards out. The hosts appealed for offside but the assistant referee’s flag was not forthcoming.
Dagnall stung the palms of Lee as the half came to a close. Mooney hesitated when another ball over the top presented the striker with another shooting opportunity and Evatt muscled in to clear the danger as the teams went in all square at the break.
The Spireites switched to 4-2-3-1. Lavery dropped in on the right and O’Shea pushed more centrally as the home side looked to build up a head of steam. Lavery twice forced Cisak in to making a save before the visitors had another sight of goal.
Cox, on the Orient left, found the run of Dagnall through the middle but the former Barnsley striker sliced an effort high and wide of the target from the edge of the penalty area. It proved to be a telling miss, as Chesterfield were soon back in front.
Lavery and O’Shea worked the ball neatly 20 yards out to fashion an opportunity for Gnanduillet that just ran away from the striker. He got to the loose ball and delivered an inviting centre Lavery was agonisingly close to and Omozuski turned in to his own net.
The home side enjoyed a spell of possession until Dagnall picked up the ball midway through the Spireites’ half and played Mooney in to the clear inside right channel. The striker took his shot early and placed the ball in to the bottom corner past Lee.
Morsy had a shot blocked by substitute Marvin Bartley before there was a long delay for the assistant referee. The Karen Child Stand did their best to get their side going and Chesterfield responded with several mazy runs but to no avail.
While, at the other end, Lee made a fingertip save to push Clarke’s header out after the Orient skipper rose highest to meet Cox’s out-swinging corner. Vincelot hit the deck in an attempt to meet the resulting flag kick but the referee waved their appeals away.
Lengthy delays disrupted the flow of the game as an assistant referee’s injury meant eight minutes of injury time, before Evatt was forced off the pitch after an aerial collision with Orient substitute Shaun Batt. Chesterfield were down to ten men and Orient capitalised.
Lee spilled Bartley’s cross in to the box and Cox was on hand to knock the ball in to an empty net and give the visitors three valuable points in their fight for survival, as the Spireites let two points slip through their grasp.
Chesterfield (4-4-2): Lee; Darikwa, Raglan, Evatt, Clucas; O’Shea, Morsy, Ryan (Humphreys 78), Duffy (Jones 76); Lavery, Gnanduillet (Harrison 81).
Subs not used: Chapman, Cartey, Banks, Dieseruvwe.
Leyton Orient (4-4-2): Cisak; Omozuski, Lowry, Clarke, Sawyer; Pritchard (Hedges 66), Vincelot, Wright (Bartley 57), Cox; Dagnall, Mooney (Batt 83).
Subs not used: Baudry, Lisbie, McAnuff, Woods.
Goals: O’Shea 19, Omozuski OG 55 (Chesterfield); Mooney 35 & 70, Cox 90+11 (Orient)
Bookings: Evatt (Chesterfield); None (Orient)
Referee: Tim Robinson (West Sussex)
Attendance: 6,524 (285).