When Knox is talking to the Danbury girl in the snow scene outside of the boarding school just before the play...when the camera looks at Knox there is snow, but when the camera snaps back to her there is no snow. There happens pretty much every time the camera snaps back and forth. But a couple times they do get it right.
Actually it's still snowing in fact it's on her clothes, it's just that with the lighting behind her it's harder to see, although you can tell the make it snow harder so you can see it around her.