Tenerife, since 1989, maritime flag since 1845.
There are quite a few legends about how this Spanish island came to have a flag that looks just like Scotland’s, but in reality it’s just a total coincidence. In 1845 Spain’s maritime provinces were assigned individual flags. These flags were just supposed to distinguish one province from another, and didn’t have anything to do with local symbolism. The designs were simple and geometric: crosses, saltires, cantons, borders, and basic divisions of the field, and they used just four colours: red, blue, yellow, and white. The Canary Islands maritime province was arbitrarily assigned a white saltire on blue. That province was split into Tenerife and Las Palmas in 1869, and the former kept the original flag.
Even though the flags weren’t supposed to represent local symbolism, many of them turned into local symbols anyway. (The Galician flag is the classic example of this.) The Tenerife flag made its way onto land, and gradually evolved from a flag representing ships from the island to a flag representing the island itself. It was officially adopted on land in 1989.