In the Revolutionary era, Connecticut claimed a huge swath of land stretching all the way to the Pacific Ocean (after hopscotching New York and New Jersey). Connecticut land developers, being an enterprising group, began selling off the choice parcels of this real estate in an area known as the Wyoming Valley, in what's now the Wilkes-Barre area of Pennsylvania. Plans were in the works to coalesce these settlements into a new state.
Pennsylvania wasn't too happy about all this, asserting that the Wyoming valley was actually in their state. The resulting conflict, called the Pennamite-Yankee War, went on for decades. Armed militia from Pennsylvania would periodically try to chase out the interlopers; shots would be exchanged and a few people were actually killed. After the American Revolution, Congress settled the matter in favor of Pennsylvania.
After getting kicked out of Pennsylvania, Connecticut continued to claim ownership of the rest of its coast-to-coast swath, focusing on portions farther west in what is now Ohio. Eventually, it was forced to abandon Ohio too. One vestige: Case Western Reserve University in Cleveland, located in the former "western reserve" of Connecticut.