It represents one of Nathan Thompson’s greatest regrets during his football career.
Now Pompey’s right-back is determined there won’t be a repeat of that painful surrendering of automatic promotion.
Thompson was skipper of the Swindon side whose blistering early-season form established them as League One pacesetters in the 2014-15 campaign.
They topped the table on January 17, 2015 – ahead of second-placed Bristol City on goal difference.
Come the season’s conclusion, though, the Robins had slumped to fourth – 20 points off City and 12 points adrift of promotion.
The League One play-off final offered another opportunity, yet a 4-0 Wembley thumping by Preston maintained the heartache.
Some three-and-a-half years on, Thompson is a key component of a Blues team currently heading League One by two points.
And he’s determined to make that advantage count to avoid a miserable recurrence
The full-back said: ‘It’s about converting how well we have started into promotion because, at the end of the day, nobody remembers if you are not promoted at the season’s finish.
‘I look back at that 2014-15 season with Swindon with a huge amount of regret.
‘Up until January, we were right in the mix and then fell away and ended up in the play-offs. We lost in the final, a tough pill to swallow, that’s for sure.
‘I don’t know whether it was a lack of experience, but the teams around us at the time were Bristol City, MK Dons and Preston, who possessed players that had been in those situations before.
‘Whereas we were a very young side at the time and unable to continue what we started.
‘Without a doubt there’s regret. At the time I was captain and to be able to win promotion with a team, at whatever level, is a huge achievement personally.
‘Missing out inspires you even more, I don’t want to get so close and not achieve it again. We’ve had a nice start and it's important we’re able to continue that.
‘I look around this dressing room and there is more experience here, there are lads who have played the level above, and hopefully that will be a huge positive later in the season.’
Thompson must sit out Saturday’s trip to Bradford after picking up a one-match ban.
Regardless, Thompson is impressed with the quality of a squad which has produced an outstanding start to the campaign.
He added: ‘We’ve got quality in attacking areas, resolve at the back, lads that run selflessly and experience. We know how to see a game out and know how to manage it.
‘There are other lads still learning. I said it last year, those that have only just come on the scene, like Ben Close, now have the experience from last season to build on.
‘Last year you would go into matches thinking “We’ll take a draw here”. While in the case of opposition like Wigan, we would come away over the moon with an away draw.
‘We’ve been disappointed with some of the draws this season, which shows a real shift in mentality.’