The Tigers were edged out of the automatic promotion places by Leicester after their 1-0 home win over Middlesbrough on Friday night but it looked like Steve Bruce's team would leapfrog the Foxes after Joe Newell's first-half own goal put them ahead.
However, Peterborough saved themselves from a 10th home league loss 11 minutes from time when McCann scored from the spot.
It meant Posh drew a league game at London Road for the first time since April 21.
Posh did not heed early warnings and fell behind in the 31st minute. Robert Koren whipped his corner in at the near post and Newell inadvertently diverted the ball past Robert Olejnik.
The hosts equalised after 79 minutes from the penalty spot after Tommy Rowe was brought down by Tigers defender James Chester.
The in-form Dwight Gayle wanted to take the spot-kick but McCann took the ball off him and stroked his shot past David Stockdale.
Peterborough survived a late scare when substitute Tom Cairney swung in a corner and Paul McShane's diving header had to be parried away by Olejnik.