Was it the final wasted review, leaving Nathan Lyon to appeal to Joel Wilson without technological oversight? Was it the missed run-out earlier the same over, Lyon dropping the ball when Jack Leach was well short? Was it the dropped catch at third man by Marcus Harris, difficult as it was? Was it the concession of 76 runs in 10 overs to the last pair, having spent all series concentrating on keeping things tight and not letting the game run away?
Was it the boundary-saving fields reflexively set for Ben Stokes and the shortish bowling that allowed him to free his arms? Was it the glaring lack of full balls at the stumps of Leach, even though Stuart Broad had just been dismissed by that very same delivery back when the game was yet to be turned on its head? Was it the mad hour with the second new ball that allowed Stokes and Jonny Bairstow to get England going in the morning? Was it the first wasted review on the third evening? Or was it, maybe, leading by 359 instead of 400?
In truth, it was all of these things. Australia, as they had so infamously done at this ground in 1981, had the Ashes on a platter for so much of the past three days, never more so than when Leach joined Stokes with 73 runs still to get. They will be haunted by it, perhaps forever, and will never again venture to England without the risk of seeing copious footage of Stokes, Leach and their own mistakes on broadcast feeds, big screens and social media.
Headingley 1981 took a long time for Australian cricket to get over. They lost the next two Tests, the Ashes, and did not win again in England until 1989. The years in between were hurtful, divided ones, and the scar of Headingley, 500-1 and all, was symbolic of much else bubbling underneath. If there is a lesson about unity in that, particularly around leadership given the complicated triangle between Kim Hughes, Rod, Marsh and Dennis Lillee, there is also the knowledge that momentum of the kind raised by Stokes can sometimes be unstoppable.
Of all the many moments leading up to final defeat, few will remain as indelible as Lyon dropping Pat Cummins' throw when Leach was beaten pointless in a near replay of the earlier run out that had taken out Jos Buttler. For all Stokes' strengths and how much they will be lauded, he has a tendency to be on the right end of run outs - of 17 he has been a part of, 14 have seen his partner dismissed.
Leach should have been a 15th, and a photo captured how, with him still metres short of his crease, Tim Paine and David Warner were running towards Lyon to celebrate a one-run victory, only the second in Test match history. But the same image also shows what Paine and Warner could not see, and Lyon had just realised. The ball had burst through his hands, trailing harmlessly away behind him as Leach scrambled back. There, more clearly than any other instant, was the Test match. The day on which Lyon overtook Lillee on Australia's list of wicket-takers will be one he does not ever wish to relive.
And yet, and yet. Two balls later, Australia still one run ahead, and Lyon used change of pace and line to fool Stokes as he went for a slog-sweep to end the bout. Pitching in line and beating the bat, it hit Stokes in front, and ball-tracking showed it to be going on to strike the stumps squarely. But Australia had wasted their final review moments earlier on a speculative shout from Cummins against Leach. In a way it was not surprising that the umpire Joel Wilson, having been the centre of attention for all the wrong reasons at Edgbaston, chose to give Stokes the benefit of whatever doubt he may have had.
"Ben was playing out of his skin. He managed to do things that you normally wouldn't and you've got to give him credit. I didn't love today but I love watching him play for that reason"
Of course, it should never have been this close, and in his post-mortem Paine tried to explain his side of things. Why, for instance, had the field been spread far and wide for Stokes, ultimately neither taking a catch on the boundary rope nor preventing the twos that allowed him to farm the strike successfully in order to face 45 balls to Leach's 17.
"If we didn't it probably would have finished a lot earlier, the way he was hitting them, to be honest," Paine said. "It's one of those things. It is a really difficult period of time to captain. I don't think anyone has done it perfectly. I certainly didn't. I don't claim to have. But when guy is going like that, you bring the field up he's hitting them for four or six anyway. What I probably would like to have done is more talk to the bowlers about their mindset.
"At times when the field gets spread they go a bit defensive. I still want our bowlers to be running in thinking about getting them out regardless of the field. But again, that's Test cricket. Ben Stokes was playing out of his skin. He managed to do things that you normally wouldn't and you've got to give him credit. I didn't love today but I love watching him play for that reason. He puts you under pressure, and he takes the game on. He looks like he plays the game the way you'd like to play and it takes a huge amount of courage to play like he did today."
Why, too, had Paine gone for the review against Leach, leaving his men exposed to Wilson's decision when Lyon pinned Stokes in front? "I've got every review wrong so I'm going to give up and give it to someone else," Paine quipped. "Patty Cummins said 'I think it might have pitched in line but I think he hit it'. And I said 'well, he definitely didn't hit it', but I was worried where it pitched. Then it was just a spur of the moment [decision] ... have a dabble at it. But, yeah, I got it wrong."
Test matches have a tendency to accumulate events in ways that leave plenty of room for reflection. Had England lost, it would have been Joe Root facing criticism for allowing Warner and Marnus Labuschagne get away against wayward bowling from Stokes and Chris Woakes in the hour after tea on day one. As it was, Paine had to answer queries on why Australia had not been able to close the game off completely from England after bowling them out for 67. Labuschagne put two excellent innings together, but no one else did.
"We haven't batted at our best, there is no doubt about that," Paine said. "But I will say I was actually really proud of the way we batted on day one. That was bloody hard work. The ball was moving around a lot, it was dark, the lights were on. I thought our guys applied themselves really well. Then I thought England probably had the best batting conditions and we bowled superbly. I wouldn't say our batting was poor by any stretch of the imagination in the second innings but what we continually did was just leave the door ajar.
"We could have batted them right out of the game and when you keep giving a team with high quality players a sniff, whether it's a Jofra [Archer] with the ball or Stokes with the bat or Root with the bat, they're going to make you pay. That was the only message we've said about batting, that we probably missed a slight opportunity to bat them right out of the game and get 450 in front. But that can happen. Sometimes you need a bit of luck.
"The boys are trying their best and as I said we feel like we've been in a position to win every Test. We're doing a lot right, we just need to do it for a little bit longer."
Finding that "little bit longer" with Headingley seared into every Australian memory is going to be a colossal task. The coach Justin Langer has been part of teams that lost dramatic games in Barbados to Brian Lara in 1999, in Cape Town to South Africa in 2011, and in Hobart to New Zealand the same year. Each time Australia won the next Test in which they played, and never were they able to enjoy the return to fitness of the best batsman in the world Steven Smith, as they will here.
To a degree, the trials of the past 18 months, ball tampering and culture, will help provide a reminder that this is a game to be enjoyed, and that being extras in perhaps the greatest and most dramatic Test match yet played is something to rejoice in after the pain fades away. But no matter how much there may be solace in how great a day this was for Test cricket, the Australians, Paine and Lyon most of all, will need to find a way to take the questions out of their minds.
Otherwise they will be like the American golfer Doug Sanders who, having missed a short putt to beat Jack Nicklaus and win the 1970 Open Championship at St Andrews, never won a major tournament. Asked whether it preyed on his mind years later, he replied: "No, sometimes I don't think about it for five minutes."