While flying to Boston when Redferne starts suspect the Warlock is on the plane Kassandra refers to Glinda from "The Wizard of Oz" as an example of a good witch but she calls her "The Good Witch of The West". Glinda's title was "Good Witch of The North".
The goof item below may give away important plot points.
Redferne's grave would not have appeared in present day, for he fell through the time warp with the Warlock, therefore he was still alive. Redferne travels back to his own time at the end of the film, in fact the swirling vortex that appears round him after he kisses Kassandra dissipates over his gravestone.