California is a fairly large landmass, big enough that you'd think no one could ever possibly confuse it with an island. Standard procedure for charting an island presumably includes "drive your boat all the way around the damn thing to make sure it isn't landlocked," and any explorer trying to mark that item off on their California checklist would notice that the Golden State is connected to the entirety of North and South America. Heck, you could stand on the coast looking east with a telescope and figure out that the Pacific Ocean doesn't make another appearance at any point in that direction.
However, during the early days of the European explorers, nobody bothered to do any of those things, because there were native peoples to rob and murder, and proper cartography simply would've taken too much time. After all, priceless religious artifacts don't melt themselves down into Spanish doubloons. Consequently, California was incorrectly drawn as an independent landmass for over 200 years.
You see, California was first charted in 1533 by Fortun Ximenez, a mutineer who broke off from Hernan Cortes' original Aztec-busting fleet. Ximenez took his stolen ship north along the Pacific coast of Mexico, and wound up landing in Baja. He decided that he and his rebellious shipmates had just discovered the Island of California, despite the fact that there was absolutely no evidence to suggest that the land he'd just stumbled upon was actually an island. Also, the Island of California was a fictional place from a famous Spanish novel, which by definition would make it difficult to locate in a hijacked galleon. At any rate, Ximenez had no time to retract or amend any part of his declaration, because he was promptly killed by natives.