There's a concept known as Schrödinger's Gun. It's kind of an amalgam of Chekov's Gun and Schrödinger's Cat.
The idea is that, until such time as you tell the players what happens, anything can be changed.
An example: In a Savage Worlds game I ran, I had a mysterious villain who was a dragon. The dragon was also an influential noble, whom the players met at a party. They immediately guessed that he was the dragon... so I changed it so that a different character, an artist I'd introduced earlier, was the dragon.
The players never knew--could never have had any way of knowing--what my original intent was. As far as they knew, the artist had always been the dragon, and they guessed wrong about the noble.