From Luke-Bones: "Dipper and Mabel secretly watch two people (about 21 years old) all out of a vortex, argue a small bit and reveal they're time travelers and work for an agency. Without noticing dipper and Mabel, one of them repars the time machine and they leave. Dipper and Mabel have no clue, but nine years later the time travelers remember back to that one day in the woods when they accidentally saw two agents that looked just like them in the gravity falls Forest. Specific, yeah. But boiling it down; twins watch TimeTravelers arrive and leave unseen. TimeTravelers were twins, only figured it out way in future."

This one should be interesting. It is rather specific but I'll do my best! This one-shot is set after Blendin's Game.

Time Warp

"Mabel, what - ?"

"Shhhh!" He sighed deeply as his sister shushed him from behind her side of the bush. "I see two people over there!"

"So? People go into the forest, Mabel."

"They just came out of that swirly thing!"

Okay, that was weird. "What swirly thing? Like, as vortex?"

Mabel shot him a "Wow, you're a nerd" look. "Uh, yeah...?"

He crouched down further and peered through the leaves. There were indeed people standing about three trees away. One of them was typing on some sort of gadget and the other was clutching what looked like a tape measure. The latter looked rather bored and appeared to be female. Both looked like they were related, with brown hair and matching black uniforms.

"What's the problem with it?" asked the woman. "Did you break it again?"

"Gee, thanks." The man shot her a glare before typing back on the gadget. "I didn't do anything. It just seems to be glitching. It's saying something about too many time anomalies or whatever."

"...So you broke it."

He groaned loudly. "No, I didn't!"

"Totally did."

"Did not!"

"Yup!"

"Ugh!" The man gritted his teeth and took the tape measure from the other. "Sometimes I wonder why I bring you with me on time missions."

"Because you love me, right, bro?"

She beamed at him as he looked up, and after a minute the messy-haired man chuckled. "Yeah, I do. Also you're better at keeping track of stuff."

"And not breaking time machines?"

"I didn't break it!"

The woman giggled at his annoyed expression and Dipper turned to Mabel. "Did you hear that?" he whispered as quietly as possible. "They're time travelers like Blendin!"

"Is it just me, or do they look kind of like us?"

Dipper snorted. "I'm sure they don't. Beside, they don't even sound like us, Mabel."

"But - "

"Fixed it! Aha!" They both turned back to the scene as the man cried out in joy. The woman snickered before quieting at his glare.

"Good job, Mr. Breaks-A-Lot. Now let's get back to the twentieth century. We've got to stop some time anomalies!"

"Yell it a little louder, why don't you?" grumbled her brother as he set the time machine (well, at least the twins assumed the tape measure was a time machine, given from past experiences) and grabbed her arm. In a flash of light, they both vanished.

"That was so cool!" Mabel jumped up cheering. "If we see Blendin again, we should totally tell him about it now that we're buddies and all!"

Dipper stood and rolled his eyes. "We're not buddies, Mabel."

"Don't burst my bubble, bro-bro!"

"Can we just go back to looking for that three-headed deer, please?"

TIMEWARPTIMEWARPTIMEWARPTIMEWARPTIMEWARP

It was nine years later when a flash of bright white filled the Gravity Falls forest. A swirling blue light appeared on a pine tree and two figures clad in black - one male and one female - emerged. As the light died down, the woman's eyes went wide.

"Wait, does this look familiar to you?"

"Huh?" Dipper Pines turned to his sibling in confusion.

"We were here before! Like, almost ten years ago! And we sat over there and watched those time travelers!" Mabel gestured over to their right where a few bushes sat.

Dipper frowned for a moment before shaking his head. "...Time travel is weird."

Yes, yes it is, Dippingsauce.