Peterborough's record-signing Tyrone Barnett marked his debut with a last-gasp equaliser to keep Doncaster at the bottom of the Championship.
A £1.1m signing from Crawley, Barnett headed home Ryan Bennett's cross in stoppage time to deny Rovers a precious three points.
Bennett was making his last loan appearance before re-joining Norwich.
Rovers had taken the lead a minute before the break when Giles Barnes headed in El-Hadji Diouf's cross.
It took Doncaster, who are now six points behind fourth-bottom Bristol City, 26 minutes to register their first shot on goal, with Kyle Bennett firing straight at Posh goalkeeper Paul Jones after good work from Simon Gillett.
Bennett then turned provider for Gillett but the midfielder took too much time to get his shot in and Mark Little blocked his effort.
More good work from Bennett enabled him to race away and pick out striker Mamadou Bagayoko, whose left-foot shot was easily saved by Jones.
But a minute before the break Diouf swung in a cross from the right and Barnes rose above his marker to head down and into the net.
Rovers almost doubled their lead nine minutes into the second half when Diouf slipped the ball invitingly into the path of Bennett, who was denied by a fine save from Jones.
Posh boss Darren Ferguson sent on Barnett for his debut early in the second half and, on the hour-mark, the striker rose highest to meet a corner but David Button tipped the ball over the crossbar.
Just when it seemed Rovers were set for a precious victory, Bennett crossed for Barnett and the big striker's header was too good for Button.
Doncaster manager Dean Saunders: "It happens in football doesn't it, but obviously the whole dressing room is gutted.
"I thought we played really well first half, I think they had one shot. We're missing chances every week and we had chances to make it 2-0 and we didn't take them.
"Luck is just not with us at the minute, even with their goal David Button has touched it on to the post and it's gone along the line and into the other side of the net."
Peterborough manager Darren Ferguson: "It was Roy of the Rovers stuff. Tyrone Barnett gives us a different outlet and that was the main attraction.
"The other thing was, when I spoke to the boy, he's got the right temperament. I had to make sure that was the case, he's a level-headed lad.
"He came on and caused them all sorts of problems. We don't have to change the way we play, the philosophy I'll never change, but I don't want him to change. We're going to have to adjust to him and not the other way around."