A fun example of an island which is not disputed: Märket is a small uninhabited island located between Sweden and Finland. It's divided between the countries and Finland even built a lighthouse on the wrong side of the border!
Another example i the island Kataja, which didn't even exist when the border between Sweden and Finland was drawn, but now sits on the border.
https://en.wikipedia.org/wiki/M%C3%A4rket https://en.wikipedia.org/wiki/Kataja