Mackay's men have now won just one in their last five in the npower Championship and from a previous position of strength are now left peering anxiously over their shoulders at the chasing pack.
Aron Gunnarsson swept the Welshman in front after 23 minutes but Posh substitute McCann equalised from the spot on 72 minutes after Mark Little was fouled by Don Cowie in the box.
McCann netted his second penalty seven minutes later after a foul by Matthew Connolly on Dwight Gayle which proved enough to send Posh out of the relegation zone on the back of six games without defeat - and complete the double over the leaders after their 1-0 win in south Wales earlier in the season.
Struggling Wolves also boosted their survival prospects with a dramatic 3-2 win over free-falling Middlesbrough at Molineux. Starting the day second from bottom, Wolves were helped on their way by a 17th minute Stephen McManus own goal before Grant Leadbitter dragged Boro level eight minutes later.
Bjorn Sigurdarson gave Wolves back a lead which Leadbitter cancelled out again, before Kevin Doyle struck to claim three priceless points for his side when he headed home a Roger Johnson cross on 70 minutes.
Sheffield Wednesday hung on for a 2-1 win over Barnsley in the South Yorkshire derby at Hillsborough despite finishing the match with 10 men, while a late howler from Brighton goalkeeper Casper Ankergren rescued Nottingham Forest a point in a dramatic 2-2 draw between the two play-off contenders at the City Ground.
Bolton's faint hopes of forcing their way into the play-offs suffered another blow as they had two men sent off in a 3-2 defeat at Charlton. Marvin Sordell and Medo Kamara had put Bolton 2-0 up before Johnnie Jackson pulled one back for Charlton, and the game changed when Craig Dawson was sent off for a second yellow card on 58 minutes.
Dorian Dervite equalised in a scramble from the resulting free-kick and Yann Kermorgant put Charlton in front from the penalty spot three minutes later. Craig Davies was the second Bolton player dismissed in injury time.
Neil Warnock's Leeds never recovered from the 31st minute dismissal of Tom Lees for a professional foul, as two goals from David McGoldrick and one for Jay Emmanuel-Thomas gave Ipswich a 3-0 win at Portman Road.
Source: PA
Source: PA