Corrected entry: Near the end of the film when Edward is carving ice sculptures in his castle, the camera pans close to one of them, making it very obvious that it is made of hollow clear plastic. The use of a fan (though not actually visible) to blow the "ice shavings" out the window is also quite evident. (01:35:05)
JC Fernandez
16th Feb 2009
Correction: You can't tell whether the blocks are made of ice or clear plastic solely by its appearance. As for the snow effect, the story is a fairy tale and doesn't conform to reality (Edward makes enough snow to blanket the town). Since as you state, the fan isn't visible, it's not a mistake.
JC Fernandez