Posh were the beneficiaries of two howlers from Preston goalkeeper Declan Rudd as North End were undone 2-0, suffering the same fate as MK Dons and Rotherham before them in crashing to a first defeat of the campaign at the hands of Ferguson's men.
And even worse was to follow as Rudd allowed a 25-yard McCann shot to slip through his hands after 29 minutes.
Rudd did atone for his errors with terrific saves from Paul Taylor and Barnett, but those goals were enough to ensure Posh closed to within a point of top spot and won eight of their opening 10 league games for the first time in their history.
Ferguson said: "It is obviously very pleasing that three unbeaten teams in a row - who will all be up there challenging for promotion - have come away from playing us with nothing.
"We have enjoyed a very, very good start to the season. Putting 25 out of a possible 30 points on the board is excellent.
"I've now challenged my players to build on this start by winning nine of the next 10 games.
"I have to admit I was a little frustrated that the game wasn't over at half-time after an outstanding first-half performance. We pressed well, passed well and were deservedly on top.
"It is very rare for a team to be dominant for 90 minutes, though, and Preston showed why they were unbeaten in that second half, but we came through it well."