Defensively-fragile Pompey self-imploded at Fratton Park as Coventry City snatched an improbable draw.
The Blues appeared to be cruising at 3-1 up after 74 minutes – homing in on victory in a rip-roaring, thrill-a-minute League One encounter.
Kenny Jackett’s men struggled at the back throughout, a frail-looking rearguard which included Rangers loanee Ross McCrorie and full debutant Sean Raggett.
Yet nobody could have anticipated the hosts capitulating in such a shocking manner against a visiting team reduced to nine men for the final 13 minutes.
Matt Godden’s penalty started the Sky Blues’ resurgence, bringing them within one goal of Pompey’s lead with 16 minutes left.
The sending off of Coventry substitute Gervane Kastaneer at least relieved some of the pressure, while Christian Burgess was brought on to see out the match defensively.
A 3-2 scoreline appeared secure – goals from Ronan Curtis, John Marquis and Gareth Evans driving the Blues towards a third-successive home win. Nothing to see here.
Defensively, however, there remained nagging fears, a weakness rarely demonstrated by a Jackett Pompey team.
Then, on 86 minutes, defender Michael Rose popped up to steer home an equaliser, drawing a cascade of boos from understandably frustrated Blues supporters.
The Pompey defence, which had lost the steadying presence of Lee Brown through injury in the first half, collapsed spectacularly against opponents reduced to a considerable numerical disadvantage.
It was agonising viewing and the Fratton faithful had every reason to vent their anger having witnessed a worrying flimsy rearguard display.
Jackett had made three changes to the side which lost 2-1 at Sunderland on Saturday – demonstrating his ruthlessness in team selection.
There would have been few shocks over the omission of Anton Walkes, eliminated not just from the starting line-up but the 18-man squad following recent below-par displays.
Yet the dropping of Burgess and Andy Cannon to the bench did represent a surprise, considering their encouraging starts to the campaign.
Burgess had been part of a defence which kept two clean sheets in four matches, while fit-again Cannon was the Blues’ man of the match in their previous Fratton Park outing.
Still, Pompey’s manager opted to reshuffle his team in search of a positive result against the Sky Blues.
McCrorie, who impressed during a 17-minute cameo as Walkes’ replacement at the Stadium of Light, was named at right-back, while Evans came into midfield and resumed the captaincy.
There was a full debut for centre-half Raggett – the Norwich City loanee breaking into the side following an injury-hampered start to his Pompey career.
Elsewhere, the sight of James Bolton among the Blues substitutes suggested he is nearing a challenge for a first-team spot following his own injury problems since early pre-season.
And while Burgess and Cannon dropped to the bench, Walkes missed out completely, not making the squad.
When the match got under way, clever play between Ben Close and Marcus Harness saw the latter’s cross from the right palmed out by keeper Marko Marosi and cleared behind.
Yet the resulting corner led to Coventry breaking the deadlock on three minutes.
Evans delivered the set-piece from the left and the visitors cleared to launch a counter-attack through Godden, who brilliantly destroyed Pompey on the break.
The striker beat the offside trap and, with only Brown covering, squared to Jordy Hiwula, who crashed an unstoppable shot into the roof of the net with the hosts still retreating.
The Blues were reeling and, moments later, Godden shot right-footed straight at Craig MacGillivray from the edge of the area.
Pompey grabbed their leveller, though, on 10 minutes, following superb play from Harness.
The winger stood up a wonderful cross from the right and there was Curtis at the far post to meet the ball with a thumping header to make it 1-1.
On 14 minutes, the hosts had strong penalty appeals waved away following Rose’s challenge on Marquis inside the box.
To supporters’ disbelief, referee Kevin Johnson refused to award a penalty – with three Blues players, tellingly, remonstrating with the official over the decision.
It was all Pompey and, on 19 minutes, Close slid the ball back to McCrorie, who fired in a goal-bound curling left-footed foot which was superbly pushed out by Marosi.
Jackett was forced into a change on 32 minutes when Brown was injured and had to be replaced at left-back by Brandon Haunstrup.
Coventry’s Fankaty Dabo earned the game’s first yellow card when he clattered into Harness after the winger had flicked the ball on.
Moment’s later, Kyle McFadzean followed his team-mate into the book after tripping Marquis off the ball during a Blues attack.
Pompey grabbed the lead on 43 minutes through a superb team move.
Tom Naylor’s sublime diagonal pass released Curtis down the left and he backheeled the ball into the path of the overlapping Haunstrup.
The left-back fired a low cross which found its way to the far post where Marquis stabbed it home to make it 2-1.
There were no changes at the interval and within a minute of the restart the Blues almost registered again.
A wonderful free-kick delivered from the left by Evans was met by Paul Downing.
But the defender inexplicably put his header wide at the far post, only for the offside flag to spare his blushes.
At the other end, Godden pounced after Downing could not fully cut out a pass – the former Posh striker seeing his angled shot magnificent saved by MacGillivray diving full-length to his right.
Pompey extended their lead on 56 minutes through the tireless endeavours of Evans.
Downing’s ball forward appeared overhit and was collected by McFadzean, only for the Blues skipper to dispossess the defender and race through to calmly finish past Marosi.
On 58 minutes Dabo received his marching orders following the awarding of a second yellow card for handball.
Yet there was controversy, with referee Johnson immediately blowing for the infringement rather than allowing play to continue, which in this instance saw Curtis sprint through to finish.
Harness almost added to the Blues tally on 70 minutes when his left-foot shot was fumbled and almost crept over the line before the keeper retrieved it.
Coventry were given a lifeline on 74 minutes when Ben Close clattered Liam Kelly for a penalty which Godden finished past MacGillivray.
Yet the Sky Blues were reduced to nine men on 77 minutes when substitute Kastaneer, who had already been booked, slid into Downing after the ball had been cleared.
There was still time for Kelly to put the ball into Pompey’s net – only for the offside flag to curtail his celebrations.
The equaliser did arrive, though, on 86 minutes, Rose side-footing home following a long throw from the right to break Pompey hearts – and initiate the boos.