Dan Butler's fatal backpass allowed Shaun Batt to score the only goal in yesterday's scrappy match at Brisbane Road.
After spending far too much time finding a parking space and being searched on the way into the ground, Pompey fans were treated to a first half where their team had plenty of the ball but failed to score.
Jed Wallace had the best chance of the first 45 minutes. Therry Racon squared the ball across the box but the unmarked teenager could only strike it straight at Jamie Jones.
Simon Eastwood did well to get down and save Lee Cook's drive from long range but that was Orient's only real chance as Pompey had more of the ball and the defence weren't really threatened.
The second half continued in the same way as the first as the referee kept stopping play for niggly fouls and the hosts barely threatened until Butler's mistake.
We shouldn't be too harsh on the young left back. His reaction told you everything you needed to know about how he felt about it and Dan has arguably been our most consistent performer since his return from Havant & Waterlooville. I'm sure he'll put it behind him quickly.
Butler cannot shoulder all of the blame for Pompey's defeat as there were opportunities to level the scores.
Patrick Agyemang and Liam Walker were guilty of being too clever with the ball in good positions and ended up having the ball taken from them. David Connolly also had a shot that crept agonisingly past the post.
There was a late rally but an awful corner from Wallace in stoppage time and a free kick in the last seconds of the game were seen out by the hosts to send Pompey to an undeserved defeat.
There were plenty of positives to take from this game. The team are generally getting better at keeping possession and dealing out danger compared to earlier in the season when a disorganised Pompey outfit would concede silly goals and wasn't able to string a few passes together.
Jack Maloney also looked bright when he came on as a substitute. He seems willing to take a man on and has a bit of pace about him. He could be an asset next season.
Hopefully the players can bounce back from this on Saturday at Doncaster.
Pompey Player Ratings
Simon Eastwood: 7 - Alert when he needed to be.
Yassin Moutaouakil: 6 - Typical solid showing.
Ricardo Rocha: 7 - Got his body in the way.
Gabor Gyepes: 7 - Won the aerial battle.
Dan Butler: 6 - Costly error in an otherwise good performance.
Liam Walker: 5 - Seems to have gone off the boil a bit.
Therry Racon: 6 - Getting better.
Johnny Ertl: 8 - Was everywhere around the pitch once again.
Jed Wallace: 6 - General play was good but finishing/final ball let him down.
Patrick Agyemang: 6 - Caused a few problems.
David Connolly: 6 - Linked up well.
Substitutes
Phil Smith
Adam Webster
Sam Sodje
Shaun Cooper
Jack Maloney (Walker 69): 6 - Looks a prospect.
Ashley Harris (Butler 77): N/A - No real impact.