Sunderland have conceded their fair share of away goals this season but few felt quite as heartbreaking as this one.
This was not a repeat of Oxford United and Scunthorpe United, the Black Cats conceding late on after indifferent displays.
Here they had to work hard to get ahead in the game but they deserved that lead.
In the first half the Black Cats had been the better side, replicating the intelligence of that impressive Doncaster Rovers win by picking their moments to commit players on the counter and produce dangerous moments on the ball.
They will rue the clear openings that they missed in what was a composed opening 45.
Charlie Wyke ought to have scored after a fine break, slicing his effort wide of the post with just the keeper to beat. So too, Jimmy Dunne, who had a free header at the back post just moments before the interval.
The second half was a more sluggish affair but when Max Power struck the ball so firmly and so precisely from the edge of the area, it looked like a pivotal moment in the automatic promotion race.
The equaliser was as frustrating as it was galling.
It was a direct ball through the middle, Sunderland unable to win the first ball or prevent Ivan Toney teeing up Matt Godden. The Black Cats had kept Godden at arm's length throughout but the arrival of such a tenacious partner in the second half finally allowed him some freedom.
It could have been worse, too.
The final moments of the additional seven saw the game swing end-to-end, both sides desperate for a winner. It was Peterborough who had the better chances, both Godden and Marcus Maddison going close.
It leaves Sunderland in a difficult position with just three games to play.
Too many draws will be the story of the season if they don't go up automatically but it is also true that this has been a punishing campaign in which five teams at the top have accumulated an impressive points total.
Charlton Athletic, currently sitting in fifth, had enough points to secure a fourth-placed finish last year even before their 4-0 win on Monday afternoon.
The margin for error has been minimal.
Sunderland found that to their cost on the Easter weekend.
Portsmouth had gone behind against Coventry City in the early-kick off and could have been out of the game by half-time. On Friday, they had benefited from an immensely fortunate late decision in their favour.
But it's true that they have responded superbly to their dip in form at the turn of the year and Saturday's game is one in which Sunderland simply have to win.
The Black Cats are outsiders for second now but it is worth dwelling on the way this race has swung so wildly from week to week.
It has been a fiercely competitive division and for all their current position of strength, Barnsley have had enough poor results and performances of late to suggest that there is potential for a slip-up.
Three wins would get Sunderland to 93 points, an impressive total that generally secures promotion.
On this occasion it might not be enough but Blackpool and Bristol Rovers will test Barnsley. It has been that way all season in this league.
Can Sunderland get there?
This was another game that in terms of the balance of the side, showed that they are in a OK place.
Though they struggled to create real openings in the second half, they played with good control and made a decent Peterborough side look short of ideas for the most part.
They have shown enough character and responded well enough to setbacks this season to offer belief that they will respond on Saturday.
They have lost the initiative and certainly, the goal they conceded was a reflection of defensive difficulties that have never been too far away from the surface this season.
If they fall short that will be the primary reason.
That particular inquest will have to wait a while, however.
There will be a twist or turn to come over the next two weeks. One of the most competitive promotion races in the recent history of this division is not going to end quietly.
You can be sure of that.