Even the experts aren't always sure which snakes are which. Particularly if it was a juvenile. Young snakes have different coloration and markings from the adult.

I caught a young snake I couldn't identify and took it to a herpetologist friend of mine who messes with local snakes. He dug out a couple of books and still couldn't tell for sure what it was. He runs a reptile shop and, just to satisfy our curiosity, he kept it and let it grow. A couple of week later it was clearly a rat snake. He offered me $20 dollars to keep it, it was worth more because it had unusual coloration. I busted the snake out of jail and let it go in the woods.