Mabel grabbed the wrong bag and ran off into the woods. Dipper didn't notice, Mabel gave Ford's rift to a Bill-possessed Blendin, and Weirdmageddon was initiated. That's the version of events in one timeline.

But, in another timeline, Dipper noticed which bag Mabel took before she could give Bill the rift. Turned to infants and taken in by the Apple family, the Mystery Twins are about to take on Equestria...

Once they're older, at least.

Happy nine years, Gravity Falls.

Part of the Multiquestria series.