As Celtic contemplate on Tuesday night's Scottish Communities League Cup meeting with St Johnstone at home, they have double cause to remember that they have already lost to the Perth side this season.
Kilmarnock's historic feat in beating Neil Lennon's side at Parkhead on Saturday - the first time they had achieved such a result in the east end of Glasgow for 57 years - was a condign reminder that when domestic duties are overshadowed by exploits abroad, there is always the chance that a levy will be extracted by the less exotic opponents.
Mind you, Kilmarnock had demonstrated that they could raise their game against Lennon's side when the Ayrshire side beat Celtic to win the League Cup for the first time at Hampden Park in March. The celebration of that accomplishment was muted even before the trophy was presented, as news spread of the fatal collapse of Jack Kelly, father of the Rugby Park midfielder, Liam.
On Saturday, Kelly's second-half penalty conversion - which supplemented Cillian Sheridan's strike before the break to ensure a 2-0 victory - was marked by the player pointing to the sky in tribute to his dad. "Yeah, obviously that's always in my mind every time I play football and it was just a great relief to score that goal," said Kelly.
"I don't want to go into too much detail about it but obviously it was nice to get this result today. Last time round we obviously couldn't celebrate as much as we'd have liked and I think it makes up for it a little bit.
"It was a great day for the boys I thought they performed excellently on the pitch. I just thought it was meant to be."
There was no doubt that Kelly was bent on taking the kick, awarded for a trip on Rory McKenzie by Emilio Izaguirre, and it was equally certain that none of his colleagues were about to dispute the issue. "Yeah, I think I just ran over and grabbed the ball, and no one was taking it off me. I wanted to take the penalty as soon as we won it. It was the first thing on my mind. I'm glad I took it and that I managed to score the goal," he said.
"To be honest I just tried to shut everything out and concentrate on the penalty. I tried not to let any outside influence affect me - the fans, the keeper, the players. I knew what I wanted to do and just concentrated, so when the time came to take it I just executed it."
Celtic had about as much life about them after they went 2-0 down as they did when they invited Kilmarnock to slug them earlier, such was the lassitude of their performance throughout. The crowd who assembled to praise them for their excellent, if ultimately unsuccessful, performance against Barcelona in last week's Champions League tie at the Nou Camp, had largely dissipated by the final whistle, but they made allowance for the effect of the team's efforts in Catalonia.
Lennon was less sanguine about the result. Having made five changes to his starting line-up, he said: "I'll certainly have a think about how I set the team out, but it was a dose of reality and that's not always a bad thing, although I would much prefer that it hadn't happened.
"To be honest, the whole thing confused me. I didn't expect us to be so flat and the first goal was shambolic from our point of view. We got what we deserved and I'll take the responsibility. I pick the team."
Lennon's opposite number, Kenny Shiels, was not constrained by false modesty. "I don't like to speak of perfection because I don't think you ever attain it, but the way we played was as near-perfection as I have seen here."
Asked if his manager was over egging the pudding, Kelly replied: "Maybe it's exaggerating a little bit but we'd been working on team organisation throughout the whole week. There was a lot of detail and we put a lot of hard work into it with a lot of double sessions.
"Celtic maybe only had two or three shots on target and for us to come to Parkhead and to see that statistic is pleasing. I thought they were fantastic in Barcelona. We shaped up at Parkhead similar to the way they did in the Nou Camp."
As for the end to a painful history of defeats at Celtic Park, Kelly said: "That was a really long record. None of the players were even alive then - well maybe Garry Hay and James Fowler!
"We need to perform like this every single week to do ourselves justice."
It may be taken as read that exactly the same message in relation to last week's outing in Barcelona, will be rendered by Lennon to his squad before St Johnstone step on to the Parkhead turf tomorrow night, hoping to emulate their own feat and that of the weekend raiders from Ayrshire.