Other mistake: Just before Hermione gives Harry and Ron the two drugged cakes for Crabbe and Goyle in the food hall, snow is falling and to a fairly low level. In spite of this, and it being a continuous shot, as the camera lowers down, the snow ceases, but right at the end of the shot, the camera view raises up a significant amount where we should be able to see the falling snow again, but someone wanted to save pennies on the VFX budget, because the snow has gone.
GalahadFairlight
6th Apr 2016