Pompey were cruelly denied a deserved victory at Carlisle after conceding in the last minute of stoppage-time
It looked like the points were heading back down to the south coast when Caolan Lavery and Marc McNulty found the target in quick succession after the break.
That turned the contest on its head after Danny Grainger had netted from the spot on the stroke of half-time.
But the United skipper ensured there would be a share of the spoils after scoring a fine strike right at the death.
Paul Cook made one change from the side that drew at home to AFC Wimbledon the previous week.
Kyle Bennett returned to the starting line-up and took up position wide on the left, with McNulty dropping to the bench.
Pompey – backed by almost 1,000 hardy travelling fans – immediately set about trying to find an early goal and it almost arrived after five minutes.
Lavery met Ben Davies’ cross from wide on the right, but his header was parried clear by Carlisle keeper Mark Gillespie.
Webster then tried a shot from the resulting corner and his effort bounced off the turf and was cleared from the line by Grainger.
The Blues were totally dominant and after Gillespie got a hand to Davies’ cross, the keeper recovered to twice prevent Enda Stevens from converting the loose ball.
And when Gareth Evans’ follow-up hit a body in the area, the visitors’ cries for a penalty were dismissed by referee Scott Duncan.
Webster surged forward and tried his luck from long-range on 14 minutes, but his shot flew over the bar.
And there were further half-chances for Christian Burgess and Michael Doyle as the Blues continued to apply plenty of pressure.
The home support were starting to grow frustrated at their side’s inability to keep hold of the ball, while Pompey passed it around neatly.
But for all of their possession, Gillespie was not exactly being overworked in the Cumbrians’ box as the final killer pass just eluded the visitors.
He had been more involved than Brian Murphy, however, with the Blues stopper a virtual spectator during the first half.
But his first task of note was to pick the ball out of the net just before the break after Burgess needlessly slid in on Grainger.
There was some debate about whether the foul took place inside or outside the box, but Duncan pointed to the spot.
Grainger dusted himself down to take the penalty and slammed it straight down the middle to leave Pompey stunned.
Carlisle United 1
The Blues continued to see more of the ball when the second half got started, but were still finding it difficult to penetrate the opposition defence.
Carlisle, meanwhile, had a chance when Alex Gilliead broke clear on 55 minutes, only for Murphy to palm his cross to safety.
Pompey had a good chance to level on the hour mark when Davies was bundled over by Luke Joyce.
The right-back insisted that the foul had taken place inside the box, but Duncan did not agree and the visitors had to settle for a free-kick.
Adam McGurk fired it straight into the wall, however, and after picking up the loose ball, his cross was claimed by Gillespie.
Cook responded with a double substitution on 64 minutes, as Evans and McGurk made way for Ben Tollitt and McNulty.
And the substitutes both made an immediate impact as Pompey turned the game on its head in the space of just two minutes.
It was Tollitt who delivered the ball that LAVERY slid in to convert at the near post to level the scores on 66 minutes.
Then, seconds later, McNULTY took advantage of some poor defending to steal possession and slam the ball past Gillespie.
It was only a brief cameo for the striker, however, as he was soon clattered while at the back to deal with a United corner and had to be replaced by Nigel Atangana.
Pompey’s patient passing game then started to come unstuck as they nervily tried to hold on for all three points.
Carlisle were understandably showing more urgency and forcing the visitors back deeper and deeper.
And they managed to snatch a leveller in the fifth minute of stoppage-time when Grainger fired impressively home from long-range.
Carlisle (4-2-3-1): Gillespie; Miller, Raynes (Asamoah 73), Ellis, Atkinson; Joyce (Sweeney 78), Dicker; Gilliead, Kennedy, Grainger (c); Ibehre (Wyke 73)
Goals: Grainger 44 (pen), 90+5
Subs not used: Hanford, McQueen, Hery, Brough
Pompey (4-2-3-1): Murphy; Davies, Burgess, Webster, Stevens; Hollands, Doyle (c); Evans (Tollitt 64), McGurk (McNulty 64 (Atangana 74)), Bennett; Lavery
Goals: Lavery 66, McNulty 68
Booked: McNulty, Lavery
Subs not used: Bass, Clarke, Boco, Chaplin
Referee: Scott Duncan
Attendance: 5,503 (953 Pompey fans)