Dipper, Mabel, Candy, Grenda, Sam, and Trixie where watching a movie at the Mystery Shack. They had hung up a white sheet and set up a projector to display the movie they were seeing. Sam sat in a chair next to Dipper, he hadn't been that interested in the movie and had fallen asleep.
Just falling asleep was a rare thing for Sam, he was a little bit of a insomniac, but perhaps fate had wanted him to fall asleep, so he could have one of his visions, and use that knowledge for the greater good.
His visions were always of Dipper and Mabel, and the adventures they had. Sometimes he sees the things they had already done, sometimes he saw things that was going to happen to them. This vision was one that had already happened.
Sam saw them at the Mystery fair, and the whole thing with the time traveler. When he woke up, the movie was almost over, and he had some questions for Dipper.
"Hey Dipper."
"Yeah?"
"Did you and Mabel ever encounter a time-traveler at the Mystery fair?"
"Yeah. How did you know?"
"I just had another one of my dreams, and it was about that."
"Cool."
"I've got another question. When you got the time machine, did Mabel say you should get two dodos, and force them to make out?"
"She did. You definitely saw our encounter with Blendin. Why'd you ask about THAT though?"
"Because I've decided that if I ever learn how to time travel, I'm going to just that."
Okay, so maybe fate didn't care too much about the greater good right then, and just wanted a good laugh.