Well, half-way through the book and only one real "error" so far (there have been a number of typos, and I was somewhat surprised by the unexpected appearance of some eels on page 141, though I suspect given the context that the phrase Gary was searching for was "prevailing mores"). 

The verifiable error pertains to the confusion about Dracula's first visit to Mina's bedroom that I referred to earlier. On page 178, Gary states that "...the key change with regard to these scenes is the number of onscreen visits that Dracula makes to Mina's bedroom. The final shooting script includes only one such visit, whereas the edited film shows two. While it is indeed possible that this change occurred during editing, there are two reasons to suspect this decision was made during the production phase." [Emphasis added.]

The "one" visit to Mina's room that Gary mentions is the one labelled as scene H-12 in the shooting script, which comes late in the film and culminates in Dracula abducting Mina and taking her to Carfax Abbey. However, Gary has apparently missed, or forgotten, that the shooting script actually includes an earlier visit by Dracula to Mina's bedroom in scene G-34, shortly after Seward has called Martin from the library to tell him that Renfield has escaped again. 

According to the shooting script, Martin takes the call from Dr. Seward in scene G-30. Scene G-31 describes Martin seeing the broken and bent bars in Renfield's room, G-32 has Martin telling the nurse about the broken bars and then exiting, and G-33 describes Dracula hypnotizing the nurse in Mina's boudoir. These last three scenes have all been cut from the Browning version, but remain intact in the Melford version. After Dracula succeeds in hypnotizing the nurse, scene G-34 reads, in full: 

G-34  INT. BEDROOM CLOSEUP BED
As nurse bends over and 
removes the wolfbane from 
about Mina's neck, Mina 
stirs restlessly, but 
does not open her eyes. 
SWING CAMERA REVERSE SHOT -
Bed in f.g. as nurse leaves bed, crosses to Dracula - 
he still stands with his cloak shielding his face
and directs her with silent
gestures to the boudoir.
Nurse exits, closing door 
behind her. Dracula starts
into f.g. towards Mina.

In the final film, all of scene G-34 has been cut until the last line (though the nurse forgets to close the door), where Dracula starts moving into the foreground towards Mina. It is this clip of film that is moved to the point earlier in the final edit of the film where it is shown just before Mina recounts her strange dream to Harker in the Seward library. 

By overlooking this scene in the shooting script, Gary is left to speculate that perhaps Browning added the scene during production because he was already toying with the idea of adding in the first visit to Mina's bedroom prior to her description of the dream, or that perhaps it was among the re-shots that were made after the production had been completed. Gary notes in chapter 6 that the bat at the window in Dracula's first visit to Mina's bedroom is a repeat of a scene from Dracula's attack on Lucy, and the portions of the scene that include the wolfbane near Mina's neck must have come from later in the film, but doesn't mention that the shot of Dracula moving into the foreground also includes the piece of cardboard on the lampshade, which means that this portion of the scene was also taken from later in the film. 

Up until the middle of the book anyway, Gary hasn't mentioned that the events from scenes G-30 through G-34 are all in Melford's Spanish-language version of Dracula. I wonder why the inclusion of the first visit to Mina's room in Melford's version didn't alert him to the fact that the footage used in the "dream" sequence in the Browning film had obviously been taken from those corresponding scenes in the English version?