What if Dipper tried to document the most mysterious, unexplained feature of Gravity Falls that is right in front of his nose: the physically impossible Mystery Shack itself? All characters are owned by Alex Hirsch and Disney, not me.
Crooked House
The video rolls, showing a hand-printed title card: "Dipper's Guide to the Unexplained #111: 'The Attic of the Mystery Shack'."
"Dipper here, documenting a Gravity Falls mystery that has been under our noses since we first arrived: the Mystery Shack itself. This building is physically impossible, and yet it exists. Mabel and I will show you."
"It's not impossible, Dipping-Sauce," said Mabel. "You're just being goofy."
"I'll prove it. First, there is a lot of extra closet space up here. Go over and stand in our bedroom door. Good. Notice that the door is set flush with the wall," said Dipper.
"Yeah, so?" said Mabel.
"I'll open the curtained closet space to the left of our door. There's a bunch of old junk in here, including a coffin in the corner, right?"
"Yes," said Mabel. "I store my sweaters in that."
"Really?" asked Dipper.
"Hey, if Grunkle Stan won't give me a big wardrobe like I have at home... You know, if we did have a wardrobe here maybe it would lead to Narnia," said Mabel.
"I'm going into that curtained space now. I'm in. Still standing in the doorway, knock on the wall just outside the door, to your left," said Dipper.
"Knock knock!" said Mabel. She knocked. "You're supposed to say 'Who's there?'"
Dipper said, "Notice, the knock came from the back of the curtained space, several feet deeper in from the door. But there's no space for it on the outside. Just the first of many impossibilities up here."
"Who's there? Mabel," said Mabel. "Now say 'Mabel who?' Come on, I can't do the jokes all by myself."
Dipper sighed, "Mabel who?"
"May Bill come in?" said Mabel.
"That's not a very funny knock-knock joke," said Dipper.
"Seriously, he's out here," said Mabel.
"Yow! D-don't do anything to us. We'll get all the evidence on tape," said Dipper.
Dipper ran to the door, pointing his video camera. There was nobody there.
Mabel giggled, "Made you look."
"Really not funny," said Dipper.
Bill (who really was there, just cloaked) thought, "They'll get in real trouble if they keep probing the dimensional anomalies here. Maybe even get trapped in another dimension like their grandfather. Should I warn them? Naah, it's more fun watching them get into messes."
"Come on, Mabel," said Dipper. "Next we'll look at why the attic stained-glass window, which is in the long wall to the left of our bedroom, looks out onto the narrow side, above the back porch. Again, no space. And that should be the wall at the opposite end from our bedroom, not at right angles to it."
"Let's probe it with my grappling hook," said Mabel.
"Actually, that's a really good suggestion," said Dipper. "We'll try that in next episode of Dipper's Guide to the Unexplained."
"They're doomed," thought Bill.