It can strike in mid-sentence, a conversation unexpectedly grounded to a screeching halt.
On other occasions, some footballing feats are engulfed by a fog impervious to close examination in the desire to gloriously reminisce.
Alan Knight’s defence inevitably focuses on his age – an ever-reliable mechanism to crank up amid a smile and a laugh.
Except the Pompey legend believes the true culprit is partial brain damage created by football.
At the age of 52, he is convinced a Blues career spanning four decades and 801 first-team appearances has noticeably impacted upon his memory capacity.
The family of the late Jeff Astle have recently won further medical acknowledgement that the former England striker died at the age of 59 through Chronic Traumatic Encephalopathy (CTE).
Rather than Alzheimer’s, it was a condition precipitated by constant heading of a football during his playing times.
Knight fears the ex-West Bromwich Albion player, who died in 2002, is not alone among former professionals.
And having suffered at least 10 concussions in action, the keeper feels he has also been struck by football’s aftershock.
‘I suffered some pretty traumatic head injuries in the time I played and believe I have a degree of brain damage,’ said Knight.
‘I have a poor memory. My short-term and long-term memory is awful – and it is awful.
‘Back in my drinking days I would probably use that as an excuse, but in the six years I haven’t drunk I still find my memory is pretty poor.
‘Sometimes I’m getting half way through a conversation and then cannot remember what I was talking about. There are also a number of matches I can never recall. I question that.
‘I think back to when I was 15 and had my first real bad concussion. The doctors were worried about possible brain damage because I was incoherent, falling in and out of consciousness.
‘I was playing for Inner London Schools against a Lanarkshire team in Luton and, while attempting to catch a ball from a corner, someone smashed me. I don’t remember anything, just black, and on the way home I was being sick in the back of the car.
‘I was kept in the nearest hospital for five or six days and they were contemplating drilling into my skull to relieve the pressure with the brain swollen.
‘I was lucky, my condition improved.
‘The after effects were bouts of pins and needles and hot and cold sweats for a couple of weeks before I played again – and through my career I would get multiple concussions.
‘But back then it was “give them smelling salts, man up and get on with it”.’
Only Jimmy Dickinson would play more games in Pompey history than Knight, who these days is the club ambassador.
It is staggering longevity from a one-club keeper whose bravery on the battlefield for the Blues often came at a high personal cost.
He added: ‘I have never really counted, but in 801 games I would say a good 10 I played on with concussion. It could have been more, I can’t put a figure on it.
‘Somebody did me when playing at Doncaster in 1981. I came for a cross and he just smashed me – but I stayed on.
‘I remember playing at Bradford once and got flipped coming for a cross, doing an aerial rotation and landing on my neck. I ended up with concussion.
‘Back then you didn’t have sub goalies. You sort of felt you were letting everyone down if you came off so, even with concussion, I stayed on.
‘One game I had to come off was against Wimbledon (April 1988) when Eric Young caught me, leaving me unconscious with a depressed left-eye socket and a broken nose.
‘The referee never even blew up, they carried on playing while I was still on the ground. I had to come off and went to St George’s Hospital in Tooting.
‘Nothing could be done at that time as the swelling needed to go down and when the team coach came to pick me up they were all moaning because they had to wait for me!’
Knight is hoping his own story and the Astle case can prompt a greater awareness about the unspoken side of football.
And he is urging the plight of former players to be given more credence by those at the very top.
He said: ‘I have suffered concussion in training on a Tuesday or Thursday and then played on the Saturday.
‘It’s the same with lads that had cortisone injections and are now suffering from joint, knee and hip problems.
‘Players now hear these stories and think you are joking. They just cannot believe the stuff you would have done to get out there and play.
‘The Pompey ex-players’ association have had a lot of funerals so far this year. Talking to lads that knew these guys, I’m told some had been struggling with dementia, but it can be easily misdiagnosed.
‘You look at boxers being pounded day in, day out, and these footballers played with laced balls often soaked with rain and they were like medicine balls.
‘Everybody says if it hits your forehead it doesn’t hurt, but that doesn’t stop you getting concussion. Your brain is obviously sloshing around in your skull.
‘And I just find it surprising the PFA haven’t followed the Astle findings up a bit more than they probably have.’