It happened the night when Dipper conjured up the undead but, the date was never really that important, it was that second that mattered the most.
Dipper is currently in a mid-summer crisis, his obsession growing every minute. He starts staring at different road than the one he is on and that is all he does for now, just stares.
An opportunity comes, a chance to find the truth. Then his Grunkle snatches his card and shot away. This is the reason why it started, why he decides to go down that road again, just long enough to show those men in suits that he is not a liar. He steals the card and contacts them. He proves himself but still fails.
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"... Promise me you'll only use it for self-defense and not go looking for trouble." Grunkle Stan asks. No, he demands.
"Okay, as long as you promise me that you don't have any other bombshell secrets about this town." Dipper bargains. This is the moment when he considers changing his path, forever, the second when the 'good boy' in him could die, the moment when something else could be born instead.
Dipper reaches his arm for a shake.
Stan stretches out his arm for the shake and uses it as a distraction. Dipper never spots the other arm bent behind his Grunkle's back and the twisted fingers it sports at the end.
They shake hands.
This was the moment when Dipper starts to walk down Stan's path and never look back.
Stan never notices Dipper's crossed fingers. Then again, why would he? Stan never saw any signs of Dipper turning into him. He never saw any signs before because there weren't any. Sure, he might have stared a few times but that was all he did before now. If only Stan had seen through Dipper's lie or perhaps if only Stan had lied better, then maybe Dipper would have never become him. Stan had missed his shots and only finds out when it is too late to change Dipper's course.