Such margins can be cruel, fine dividing lines brutally decisive.
The outside of a post, the flashing half-volley possessing the slightest of deflections to divert it over the crossbar.
And the defender unable to convert from close range following a goalkeeping intervention.
Pompey’s opening 20 minutes promised much, daring to suggest the drawing of first blood in the maiden south-coast derby for seven-and-a-half years.
The opportunities were there, created through an exhilarating opening to the match, relentlessly roared on by a vocal Fratton faithful.
This was the Blues’ chance, their Premier League hosts struggling to engineer a foothold as they backpedalled towards the Milton End.
Then, on 21 minutes, the door was bolted shut.
A classy finish from the outstanding Danny Ings, pure top-flight calibre, broke Pompey’s heartening start – and shattered their hearts.
Kenny Jackett’s side’s purple patch was over and with it the Carabao Cup tie was effectively settled.
It can be speculated what could have been had John Marquis not struck the post, had Brett Pitman not narrowly shot over, had Burgess pounced from a few yards out.
As it was, they couldn’t capitalise in such promising positions – and Ralph Hasenhuttl’s side seized control.
The second was registered on the stroke of half-time, through that man, Ings, punishing a mistake from Ronan Curtis in the build-up.
Finishes from Cédric and substitute Nathan Redmond in the final 13 minutes added further gloss to proceedings, a callous 4-0 scoreline with cold-eyed ruthlessness.
The side residing 13th in the Premier League were widely anticipated to defeat a club two divisions below and entrenched in lacklustre form.
The end result certainly backed such expectations, Ings a magnificent presence and Southampton discovering a swagger as the second half wore on and the Blues faded.
Yet there had been hope, wicked belief this occasion would see someone join entries such as Yakubu, Lomana Lualua, David Norris, Joel Ward and Barry Horne in Fratton folklore.
It lasted 20 minutes – then Ings intervened in Southampton’s first attack of the match.
Ultimately, the League One outfit were crying for the final whistle to avoid further mutilation.
Still, regardless of the eventual gulf in quality, the Fratton faithful ended the game dominating the atmosphere, singing louder than those in the Milton End celebrating their warranted victory.
It represented only Southampton’s second Fratton Park win in more than 43 years, deservedly so in the end, nobody could begrudge them that.
But for those opening 20 minutes, it could have been an entirely different outcome. Games turn on such incidents.
Jackett was delivered a pre-match boost when Tom Naylor was declared fit yesterday morning.
The skipper was rated as 50/50 on Monday having collected two different injuries in Saturday’s 1-0 defeat at Wycombe.
However, he was able to retain his place in the centre of midfield alongside Ben Close, with the Blues instead making two changes elsewhere.
Of the side which suffered that abject loss at Adams Park, Ellis Harrison was missing through suspension, while Andy Cannon dropped to the bench.
Instead Brett Pitman and Ryan Williams earned recalls, lining-up in the attacking three alongside Curtis.
Meanwhile, Oli Hawkins continued his central-defensive role after impressing in his new position at the weekend.
There was no starting spot for Gareth Evans, who was named on the bench alongside Isle of Wight youngster Leon Maloney.
Meanwhile, Marcus Harness, Lee Brown, Bryn Morris and Jack Whatmough continue to be sidelined by injury.
Amid a deafening atmosphere from kick-off, both teams began to feel their way into proceedings as the rain fell.
Then, on nine minutes, Williams fed Marquis down the middle and his powerful early shot kissed the outside of the post.
Moments later, more good work from Williams down the right saw Marquis given a sniff inside the box but James Ward-Prowse hacked the ball out for the game’s first corner.
That was delivered from the right by Brandon Haunstrup but it was comfortably cleared by the visitors.
Pompey were on top and, on 13 minutes, Pitman flashed a fierce first-time left-footed shot over, although it must have taken a slight nick, with a corner awarded.
The pressing of Naylor then dispossessed Pierre-Emile Højbjerg inside his own penalty area on 18 minutes, winning another corner down the right.
Haunstrup’s set-piece was brilliantly stopped by the fingertips of Alex McCarthy but Burgess was unable to force it over the line from close range and the keeper saved.
The deadlock was broken against the run of play by Southampton on 21 minutes. Firstly, Ings’ drive was beaten out superbly by Craig McGillivray.
But when Ward-Prowse retrieved the ball, he teed up the striker for a second attempt, which was placed low into the far corner.
Moments later, MacGillivray had to be quick off his line to deny Ings as the linesman’s flag stayed down, with Hawkins’ desperate block preventing Højbjerg’s follow-up finding the empty net.
On 35 minutes, the Pompey keeper denied Ward-Prowse, saving his stinging shot from outside the box.
The visitors extended their lead on 44 minutes after Curtis carelessly gave possession away in his own half.
That sparked a Southampton attack, with Michael Obafemi sliding a pass across goal and Ings cleverly nudging the ball past the advancing MacGillivray.
During the interval, former Pompey players Norris, Ward, Adam Webster, Greg Halford and Benjani Mwaruwari were introduced to the crowd – and certainly well received.
There were no substitutions at the break and, on 48 minutes, Haunstrup pulled the ball back for Curtis, who was crowded out.
Hawkins collected the game’s first booking on 53 minutes following a tactical foul on Ward-Prowse as he threatened down the middle. He was quickly followed into the book by Pitman.
Pompey were forced into their first substitution on 59 minutes when Hawkins had to come off with an injury. He was replaced by Paul Downing.
The Blues were still searching for a way back into the game and on 73 minutes Pitman helped the ball on to Curtis, whose snapshot took a deflection for a corner.
It was 3-0 on 77 minutes, when the ball was cut back from the byline and Cédric slotted a shot past MacGillivray.
Redmond made it 4-0 three minutes before full-time as Southampton continued to pour forward sensing more blood.
What could’ve been – yet Southampton’s final victory could not be begrudged, regardless of the occasion.