Leyton Orient missed the chance to return to the top of League 1 after two late goals fired Peterborough to a 2-1 win at Brisbane Road.
The hosts had to wait patiently for the opener but it arrived in the second half when David Mooney headed in his 17th goal of the season.
But Britt Assombalonga headed home in the 88th minute and the visitors stunned Orient when they netted the winner in injury time through Tommy Rowe's fierce strike.
The O's handed a debut to goalkeeper Shwan Jalal after he joined yesterday on loan from Bournemouth. He was brought in after Ben Alnwick was ruled out indefinitely due to personal reasons.
Mooney made his first start since December, in place of Robbie Simpson, while Kevin Lisbie was fit to be named on the bench.
The visitors had the first effort on goal but Jalal comfortably saved Danny Swanson's shot. Orient responded and went close through Dean Cox but his strike was deflected just wide.
Peterborough made a bright start and had another opportunity through Rowe's header but it was straight at Jalal.
The Posh were looking dangerous and Nicky Ajose was a real threat but his volley was just too close to the O's keeper.
Lloyd James had the chance to find the target from outside the area but he struck his shot over Bobby Olejnik's bar.
Orient came so close to grabbing the opener when Cox volleyed Scott Cuthbert's cross against the post.
The hosts ended the first half strongly but Mooney's curling shot floated just past the far post as it remained goalless.
Peterborough started the second half with purpose and went close through Rowe but his long-range strike was tipped away by Jalal.
Elliot Omozusi was forced off with an injury and the defender was replaced by Gary Sawyer.
The O's had a let off when Assombalonga's shot was pushed onto the post by Jalal and Rowe fired the rebound wide.
Russell Slade made a second change when he introduced striker Lisbie in place of Chris Dagnall.
The O's grabbed the opening goal in the 70th minute when James sent in a superb free-kick and Mooney headed the ball into the net.
Peterborough very nearly equalised immediately but Ajose's effort flew inches past the post.
The final Orient sub came when Simpson replaced goalscorer Mooney with less than ten minutes remaining.
But the visitors grabbed a late equaliser when Assombalonga headed the ball home from close range.
It got even worse as Peterborough struck an unlikely winner in the 90th minute as Rowe's powerful effort sent Orient to their second league defeat in a row.
Leyton Orient: Jalal; Omozusi (Sawyer, 56), Clarke, Baudry, Cuthbert; Cox, Vincelot, James, Odubajo; Mooney (Simpson, 84), Dagnall (Lisbie, 67)
Subs not used: Larkins, Lasimant, Bartley, Ness
Peterborough: Olejnik; Alcock, Knight-Percival, Brisley, Little; Rowe, Payne (Washington, 73), Bostwick, Swanson; Ajose, Assombalonga
Subs not used: Day, Ntlhe, Kearns, Newell, Anderson, Vassell
Referee: Lee Probert