It was the one that got away. Of all the failures and all the disappointments, it was the one that hurt the most and that still rankles to this day.
What is done is done, of course. Michael Beale cannot rewrite history but he can shape the future as he prepares for another Hampden head-to-head with Celtic.
The build-up to the Viaplay Cup final next month will naturally involve talk of the last time the Old Firm met when the silverware was on the line. Beale needs no reminding of it more than three years on.
On a day that Rangers dominated, Celtic celebrated. There was no technology to determine that Christopher Jullien's strike was offside and should not have stood and an Alfredo Morelos penalty miss summed up an occasion to forget on what could, and should, have been one to savour.
“Yeah, they ended up with the trophy," Beale said as he reflected on the 2-1 win over Aberdeen and cast his mind back to his first Hampden final. "Well, if we had VAR... Are we using VAR at Hampden for this final?
"That was a sore one at the time. After that game, it was more frustration because we had played very well. That's a trophy that they've got that's a disappointment for us.
"Look, I'm just delighted that we have come back to another final. A lot has been said about this group but all they can do is keep winning football matches - and they keep finding a way. That's the sign of a good team."
The win over Aberdeen on Sunday was the most significant of Beale's tenure as Rangers manager. If it can be topped next month, the Englishman will have his maiden medal as a boss in his own right and will have made the first inroads into righting the record that plagued his time alongside Steven Gerrard.
The failure to win a knock-out trophy was the black mark on record of the management team that delivered title 55 and restored Rangers as a name and a force in Europe. Just weeks after his return to Ibrox, the silverware is within reach for Beale.
Success is ultimately what he will be judged on. As he embraced his players and applauded the supporters at the end of an extra-time victory, Beale was more concerned about what the achievement and the occasion signified for others.
"It means everything to me, but it's only a small thing when I look at all of those fans and the players as well," Beale said. "I think it means a lot to the group. I'm delighted for them.
"For me, as a coach, at the end of the season I'll look back and see if I've done a good for Rangers and then I'll bite down the gum shield and go again for 12 months. I really feel the responsibility, of wanting to do well for other people rather than myself."
Beale has spoken about being performance happy and results happy in the weeks since he succeeded Giovanni van Bronckhorst. He is still searching for the complete showing, but this win - secured thanks to goals from Ryan Jack and Kemar Roofe - was another triumph to go alongside the handful that have been earned in the Premiership.
Given their advantage in terms of personnel and the scoreline, the closing stages were perhaps more fraught than they needed to be as Aberdeen attempted to salvage their cup dreams while captain Anthony Stewart sat in the stands and contemplated his red card moment of madness.
This was a win that will do Rangers the world of good in terms of confidence and the buy-in to the Beale blueprint. It will also further allay the anxiety that the Englishman has addressed on a couple of occasions.
"We have to keep talking about it," Beale said. "It's natural when you try to hold on to something, you drop deep.
"On Sunday, I thought we were in control of the game. We started well, we should score with Fashion. Aberdeen were always in the game, I'm not saying we were dominant, but we were the stronger team in the game.
"We had the ball, we had chances, we had good areas to cross and then, from once isolated moment, we've conceded. And you saw it at a couple of set plays, when we are looking a little bit anxious.
"And even then, at half time, we spoke about getting ourselves in front, we're again really dominant, then Allan has to make a big save out of nowhere. There are some things for me to work on.
"It's fair to say that in the time I've been back in, we've had more games than training sessions, in terms of we've been at it every two or three days. We will be until another six or seven days time, but at this moment in time, their reaction to the advice I'm giving, the people coming off the bench, is fantastic.
"The game's not always plain sailing because two teams play it. In this moment in time, I think that's a really good Aberdeen team. It's young, it's got legs, it's trying to play the right way.
"I know Jim will be disappointed tonight but he can be pleased with the direction they are moving in. I want to give them some credit as well."
Aberdeen emerged from a compelling cup tie with a pat on the back but it was Rangers who merited the victory and who can now look forward to a return to the National Stadium.
It was done the hard way after Beale side's recovered from the loss of the opening goal to Bojan Miovski just before the break. Not for the first time, the manager had his say and earned his win with his messages at the interval and changes during the match.
"It's a group of which 75 per cent I've worked with for a long time, so there's a huge amount of trust in some of the advice I give them," Beale said. "I think that's a good sign, that you go in and are able to talk things through and stay on it.
"But, I think also, you're doing me more service than maybe I deserve. I think sometimes the game has just got to settle down. Maybe teams get tired later on and maybe our style just gets them.
"I think it was a much more mature performance than when we lost to Aberdeen maybe a few years ago, when in the second half we rushed the game and played more long balls.
"On Sunday we kept playing. And at times our fans want us to play a little bit quicker, but when it nearly comes off - we might have a fantastic move for fashion to have one, and Tav to hit the bar.
"If one of them goes in, we're talking about a wonderful goal with lots of passes. So, I'm seeing really good signs.
"I was pleased with a lot, but for a long period of the game you are losing and there's that horrible feeling in your stomach, but fair play to the players, they keep finding a way."
