Pompey conceded two goals in the final five minutes as victory turned into defeat against Brentford.
Guy Whittingham’s men appeared to be headed for a 2-1 win – potentially capping off a momentous week after the Pompey Supporters’ Trust secured ownership of the club.
Then Bradley Wright-Phillips netted in the 85th minute following a goalmouth scramble.
And Clayton Donaldson sealed a stunning late fight-back for the promotion-hunting hosts by galloping through to net a minute later.
Earlier, Wright-Phillips had given the hosts the lead in the 21st minute after Simon Eastwood failed to get enough behind a punch.
A classy swivel and shot from David Connolly just four minutes later drew the visitors back on levels terms.
Then Shaun Cooper drove in a magnificent 30-yard effort in the 58th minute to send the 1,796 Pompey fans wild.
But two late goals swung the match Brentford’s way, as Whittingham’s side lost for only the third time in 13 matches.
Whittingham made three changes to his starting XI, with Sam Sodje, Dan Butler and Patrick Agyemang coming into the side.
They replaced hamstring injury victim Ricardo Rocha, Liam Walker and John Akinde, with the latter two dropping to the bench.
When the match got under way, Eastwood came to Pompey’s rescue in the 10th minute with a superb stop.
Saunders’ fierce free-kick from the left was touched on to the crossbar by the keeper and ricocheted to safety – much to Blues fans’ relief.
Brentford were doing all the early running and broke the deadlock in the 21st minute when Eastwood failed to get too much distance on an attempted punch clearance under a challenge from Donaldson.
The ball fell to Wright-Phillips and he lashed the ball home, with Eastwood out of position.
Within four minutes, though, Pompey were level with a classy finish from Connolly.
Harlee Dean missed his clearance and the ball fell to the striker who, with back to goal, swivelled and drove home a first-time left-foot shot.
It was 1-1 at half-time, but in the 58th minute Cooper gave the Blues the lead.
There appeared to be little danger when the loanee picked the ball up 30 yards out.
But he took a touch before unleashing a right-foot drive into the far bottom corner.
Donaldson had a glorious chance to equalise in the 69th minute with just Eastwood to beat, but the keeper saved his shot with his legs and Sodje cleared for a corner.
But in the 85th minute the Bees levelled – Wright-Phillips drilling home, before Donaldson sealed the win a minute later.