Switzerland consists to large parts of regions and cities that „flipped“. Even in the 20th century, an Austrian Region wanted to join (and had a vote about it that said yes), but Switzerland and Austria denied. The Italian province of Lombardy still has a movement that wants to join. Similarly, Alto Adige has a political movement to join Austria. So it is still happening today on older and more recent borders. Secession movements are even more common.
Maybe places like Transnistria and Artsakh, not officially recognised, but not governed by the country that they officially belong to.