Why bitter rivals are always up for a fierce showdown
God knows what was going through the mind of the person who spat on Karl Lacey, Donegal's injured defender who was walking back to the Healy Park dressing room during their league defeat to Tyrone in 2013.
It was a vile act towards the reigning Player of the Year, on an afternoon that had a desperate edge to it. Afterwards, Mickey Harte went to apologise to Lacey on behalf of Tyrone GAA, but that wasn't what Donegal manager Jim McGuinness took away from that brief talk.
Instead, it was a line from Harte recounted in McGuinness' autobiography when they shook hands and Harte said: "You were great All-Ireland champions."
The implication of the word 'were' drove McGuinness mad.
If you thought that was the low point and things couldn't get any worse between these two counties after that, then you were badly wrong. Back to that in a minute.
The two sides meet tomorrow in the final group clash of the Super8s. They have never faced off this far into the All-Ireland competition. Clubs cannot match the demand for tickets. It is, simply, the biggest game Ballybofey has hosted.
Some might trace the enmity back to the infamous 'Battle of Ballybofey' in 1973, when Donegal's star of the time, Neilly Gallagher, almost lost an eye after a punch from his marker, Mickey Joe Forbes, with reports of stones and bottles being flung at the dugouts.
McGuinness was determined to change the relationship from his playing career, which was of Tyrone supremacy.
Prior to the very first challenge match he had as manager of the county Under-21s against Sligo in Ballyshannon, he told the story of the time he played for the Under-21s at full-forward against Tyrone in Castlederg. His marker beat him to the ball and put his boot into his face in the process, laughing heartily when the referee did nothing about it.
"The point was that we held Tyrone and teams like that in the height of respect and they regarded us as players to be toyed with, to mash down," he recounted.
"Tyrone and Armagh had been driving me demented for years, but they weren't gods. They were not superhuman. People in Donegal thought they were. We elevated them in our minds and they regarded us as a joke. They knew we would always buckle in the minutes that mattered. The inference was there."
In 2011, Donegal met Tyrone in the Ulster Championship. The night before, in a meeting room in the Slieve Russell Hotel, McGuinness spent two hours dissecting Tyrone - right down to the type of sledging they would try.
In that Ulster semi-final win, the relationship flipped. Tyrone misread the signs.
When they met again in 2012, Harte's message to his players was not to respect their opponents, but to ask, 'What have they done in the game?'
Another defeat, on the way to Donegal's All-Ireland title, drove Tyrone mad.
Donegal defender Eamonn McGee recalled: "They were a means of testing ourselves and it fed into the narrative that Jim was telling us. And we bought it. They looked down their nose at us.
"It wasn't a case that they actually did look down their noses at us, we were never on their radar. But it was the way that Jim sold it to us. We were eager and easy to take it all on board and believe it.
"The main thing with Jim was that they had looked down their nose at us in the previous 10 years and that was his big, big point."
Onto 2013. After the league defeat, McGuinness kept a couple of ploys from sight in Healy Park.
Tyrone's young goalkeeper Niall Morgan starred that day, saving a penalty from Michael Murphy but also hitting two frees and a '45.
Tyrone had to go to Ballybofey for the Ulster Championship after an unsuccessful attempt to have it moved to Clones. When Morgan stood up to kick his frees, he found himself faced by four men, all with their arms upright, encroaching on his space, giving him a load of verbals. He crumbled.
"Jim had the Championship encounter in mind during the league encounter, so he was able to learn plenty from that, whereas other managers would just be going hard for that league game," explained McGee. "We wanted to win it but he didn't see it as the be-all and end-all. He learned a good bit and it was one of the things we targeted, Morgan."
As bad as things were in 2013, the 2015 meeting was a mess. Foul-mouthed sledging went on throughout and a melee at half-time at the narrow tunnel going underneath the stand made the whole thing so, so ugly.
Joe McMahon, who started on the bench for Tyrone that day, said: "Right from the get-go, when you arrive there, that small, poky changing room, it's far from the luxury of Croke Park or Healy Park.
"You are on top of each other, and with the size of the panel, the backroom team, you are just itching to get out on the pitch.
"It was difficult to control the psyche at the time. Stepping out onto that pitch was a cauldron. The atmosphere was almost different than I had ever felt.
"Because they were on top of you, it was so personal. You could hear everything said to you by the opposition supporters."
His brother Justin was marking Michael Murphy. The ball wasn't much of a concern to Justin, who was doing what Kerry's Aidan O'Mahoney had done in the All-Ireland final the previous year, but the locals didn't take kindly to their chieftain being restrained on his own premises.
"They were shouting at him, calling him a thug," recalled Joe.
It was the lowest point. At half-time, Donegal manager Rory Gallagher went to make a point to the referee. McMahon stepped in and had his say.
McGee recalled: "And the next thing you know, me and Gavin Devlin are shouting at each other.
"There was a stage where that whole relationship wasn't healthy, in terms of a sporting context. I wouldn't say 'hate', because I never hated a team, but it was only a small level below hate. It was so toxic and nasty."
After two years out of county football, the Gaoth Dobhair man feels differently about it all.
"You get older, a bit wiser, and you start to think, 'Why were you at this?' It's only when you come to the end of your career that you start to realise all these things," he said.
McMahon added: "Looking from the outside in, we were aware they enjoyed themselves a bit more than a county team should. We fed off that."
So much has changed in the last decade. Tyrone have won the last two encounters. But Donegal have been unbeaten at the ground since 2010. Something has to give. It always does.