A goal in the 94th minute from Genserix Kusunga condemned Posh to a 5-4 defeat at Boundary Park.
Peterborough United threw away a 3-0 and a 4-2 lead to suffer a last minute defeat at the hands of Oldham Athletic with the Latics recording a 5-4 win over Darren Ferguson's men who had Lee Tomlin sent-off for handball on the line.
Posh had a 3-0 lead at half-time thanks to goals from Tomlin, Britt Assombalonga and Tommy Rowe, but a goal inside the opening two minutes of the second half set the tone for a 45 minute period at that the home side dominated from start to finish.
Oldham worked an opportunity inside the opening 25 seconds as he ball fell to Charlie MacDonald, but he was leaning back and sliced his shot wide of the target.
However, Posh, after being on the back foot for much of the opening exchanges, found the breakthrough on five minutes as a cross from Little was glanced past Oxley by Britt ASSOMBALONGA for his 22nd goal of the season.
Genserix Kasunga produced a fine block to deny Danny Swanson's goal-bound shot from 10-yards after good work from Assombalonga with Rowe heading the resulting corner kick wide of the far post.
On 21 minutes, Swanson wasted a golden chance to make it 2-0 after excellent approach play from Tomlin and Ajose, the Scottish midfielder curling wide with his left-foot.
Posh made it 2-0 on 24 minutes as a corner kick from Tomlin was met by Tommy ROWE at the near post who planted a header past Oxley and into the back of the net.
Tomlin saw a free kick from a wide area tipped over by Oxley on 39 minutes after Knight-Percival was felled by Smith.
In one-minute of added time, Posh made it 3-0 as Swanson's shot was blocked and the ball fell kindly for TOMLIN who fired low past Oxley and into the back of the net.
Oldham made a great start to the second period, coming close to reducing the arrears through HARKINS free-kick which was beaten away by Day before reducing the arrears when the same player fired home following a scramble after a corner was only half cleared.
Clarke-Harris then fired over as Posh began to wobble. That wobble intensified on 54 minutes as the Latics scored again, this time WORRALL with the finish, side-footing the ball into the top corner after Harkins had beaten Little by the by-line.
There was a disaster for Oldham on 62 minutes as a throw in from Grounds, which was meant for the goalkeeper Oxley was intercepted by AJOSE who finished clinically from 10-yards to restore a two-goal cushion.
Jermaine Anderson replaced a limping Payne on 63 minutes. Hawkins fired wide on 66 minutes after cutting inside from the left.
Moments later, Oldham reduced the arrears to 4-3 as the languid Harkins fed Grounds and his low cross was converted past Day at the near post by former Posh midfield man James WESOLOSKI.
Day did well to parry a Worrall effort after the home debutant was left in acres at the back post. Harkins curled a left-foot shot over the bar on 74 minutes as Oldham piled forward.
Ajose headed a Tomlin free-kick over the bar on 78 minutes. Tomlin was then denied by Oxley after a good run and toe poke finish.
On 88 minutes, Posh were reduced to 10-men as Tomlin handled a goal-bound header on the line and HARKINS stepped up and sent Day the wrong way from the spot.
Deep into stoppage time, Oldham completed the misery for Posh as centre back KUSUNGA smashed home from close-range after a corner was only half-cleared.
Final Score: Oldham Athletic 5-4 Posh
Oldham: Oxley, Grounds, Wesolowski, Kasunga, Smith, Tarkowski, Dayton (sub Brown 33min), MacDonald, Philliskirk (sub Clarke-Harris 33min), Worrall, Harkins. Unused subs Mellor, Winchester, Rodgers, Lockwood, Rachubka.
Posh: Day, Alcock, Bostwick, Knight-Percival, Little (sub Brisley 57min), Rowe, Swanson, Payne (sub Anderson 63min), Tomlin, Assombalonga, Ajose. Unused subs: Olejnik, Kearns, Vassell, McCann, Newell.
Attendance: 3,571 (273 Posh fans)