With the aid of a geographical map, Ann Althouse makes a plausible argument that Michigan's Upper Peninsula (a.k.a "da U.P." -- where the inhabitants call themselves "Yoopers") appears to have been stolen from Wisconsin by Michigan:

That is ours, baby. It's like that mitten is reaching up there an yanking off our manhood. And speaking of manhood, I think, if we had that peninsula that is rightfully ours, the politics of Wisconsin might shift from Mommy Party to Daddy Party.

But does the peninsula rightfully belong to Wisconsin? I think a better case could be made that the entire state of Wisconsin actually belongs to Michigan, for it was once a part of the Michigan Territory:

The Territory of Michigan was an organized incorporated territory of the United States that existed from June 30, 1805, until January 26, 1837, when the final extent of the territory was admitted to the Union as the State of Michigan. Detroit was the territorial capital.

The Michigan Territory only got bigger, until it engulfed and devoured Minnesota, Iowa, and nearly half of the Dakotas:

In 1834, all of the lands acquired in the Louisiana Purchase that were as yet unallocated and lay east of the Missouri River (generally, the Dakotas, Iowa and the western half of Minnesota) were attached to the Michigan Territory, an area that was officially characterized as "north of Missouri and east of the Missouri and White Earth Rivers." At this point, Michigan Territory included what is now the states of Michigan, Wisconsin, Iowa, Minnesota and a large portion of the Dakotas.

It was not long after that that Michigan went to war with Ohio over the Toledo Strip, but Michigan lost. President Jackson favored Ohio and in a compromise deal, it was decided to allow Michigan to become a state without the Toledo Strip but including within its new state borders the entire Upper Peninsula.

Michigan became a state in 1837, shortly after Wisconsin was made a territory:

Wisconsin Territory was established in 1836 with the present boundary in the Upper Peninsula.

the 1836 Wisconsin Territory included all of the present-day states of Wisconsin, Minnesota, Iowa, and that part of the Dakotas that lay east of the Missouri River. The portion of the Territory east of the Mississippi River had originally been part of the Northwest Territory, which had itself been included in the cession by Britain in the 1783. Most of the remaining land of the original Wisconsin Territory was originally part of the Louisiana Purchase, though a small fraction was part of a parcel ceded by Great Britain in 1818. This land west of the Mississippi had been split off from the Missouri Territory in 1821 and attached to the Michigan Territory in 1834. In 1838, the Iowa Territory was formed, reducing the Wisconsin Territory to the boundaries for the next ten years; upon granting statehood to Wisconsin, its boundaries were once again reduced, to their present location.

Interestingly, Michigan was not always pictured as a mitten. In older maps (like the 18th Century Mitchell Map), it looked more like a defensively angry shark, butting its snout into an Upper Peninsula which had a far more offensive overhang than it does today.

Not to sound like an apologist for a bullying, hegemonic state, but I can see why people in those days might have been led by that map to conclude that the poor shark needed to protect itself from the northern menace!