Just one of the truisms Celtic players are reflecting on today as the dust settles on the major collapse of their defensive reputation in Warsaw on Wednesday.
What had appeared to be the strongest element of the side buckled to the point where next week's return leg in the qualifier against Legia came perilously close to being filed under irretrievable.
Conceding four goals - plus two unconverted penalties - had shades of how Barcelona cut through them in the Nou Camp to win 6-1 in December.
But the Pepsi Arena is not the Nou Camp, and Legia are certainly not Barca.
The damage was done not by Neymar or Iniesta, but by the Celtic players themselves.
The fact they were reduced to 10 men for half the match following Efe Ambrose's red card should not be allowed to serve as mitigation for individual and collective negligence, the price of which was reflected in the final scoreline and the task which now faces the Hoops if they are to make it into next Friday's draw for the play-off round.
It is fine for manager Ronny Deila to hold his hands up and admit he got it wrong. With his team selection he certainly did.
But the players - especially the more experienced ones - must also take responsibilities for their actions, or, at times, inactions, on a night which should haunt them forever.
To his credit, defender Mikael Lustig did not attempt to downplay the seriousness of their failings, and admitted that, but for some crucial saves by Fraser Forster, this could have been another contender for a club record-equalling defeat in Europe.
He said: "Absolutely, it could have been. We need to sit down and look at it. It was too easy [for Legia].
"We got the perfect start, 1-0 with an early goal away from home.
"But especially those of us in the back four should be experienced enough. We have been here before.
"We should have maybe played it a bit simpler. But we were losing balls in midfield, and then they scored.
"It was the same with the red card. If we lose the ball there, it's going to be dangerous.
"Of course the red card played a part in it as well. But, it was a lot of mistakes."
Legia had clearly studied how Deila wants his side to play, building from the back, and put pressure on the defenders and sitting midfielders in possession, forcing errors and creating turnovers in dangerous areas while preventing the Hoops enjoying the time or space to play accurate passes forward.
It was a predictable tactic from the Polish champions, and one the Celtic players should not only have anticipated, but have handled much better.
Lustig said: "They were playing at home, so of course they are going to go forward.
"But when we scored the first goal, we needed to be smart.
"We know straight away when you score a goal it is crucial to be thinking.
"I don't know if there was a lack of concentration at their equaliser. But you need to show more experience."
The Champions League qualifiers are a defining series of matches in any season for Celtic, but particularly so when a new manager has just moved into the job and is trying to impose his style on the team.
Deila is now finding out the pressure that comes with the salary, and Lustig does not shy away from the gravity of the situation.
"We have put ourselves in a really bad position now," he conceded before looking for a positive on which to pick himself back up, and adding: "But we did that last year against Karagandy.
"We needed to win that game 3-0, and we did it. So we know we have done it before.
"But right now, we are really disappointed - and we should be really disappointed.
"It was really tough to take."
He went on: "We just need to get through, and how we do it doesn't matter.
"But first of all, we can't play football like we did on Wednesday."
If they can't undo the wrongs of Warsaw, the Europa League play-off round awaits.
That might be considered some consolation, but not for the Swede who is not just referring to the £20million hole which would appear in Celtic's turnover if they drop to a competition which returns only about a 12th of what can be earned for taking part in the group stage of the big one.
"It's the Champions League this club lives on," said Lustig, mindful of how uncompetitive the SPFL has become.
"We have done it for two years in a row, reaching the group stage.
"The club ands the fans deserve Champions League football.
"But it is up to us now to make it through."
Even this early in Deila's Celtic career, a defining moment has arrived. His players owe it to him to help him make it a successful one.