The character of Madame Ouspenskaya (Gypsy Woman) was named after Maria Ouspenskaya, who played the character "Maleva" in The Wolf Man (1941) and Frankenstein Meets the Wolf Man (1943).
When Count Dracula and Abraham Van Helsing first met in the movie, Van Helsing asks him if he descended from Vlad Tepes, the first Dracula. It's a reference to Vlad III. Draculea, also known as Vlad the Impaler. He was a Voivode of Vallachia, a former region in south Romania, from 1456 to 1462. Bram Stoker used this person to create his character Dracula.
When Dracula visits Renfield in the sanitarium, he pulls apart the metal bars on Renfield's window to have him crawl out. When Dracula let's go of the bars, it is obvious that they are made of rubber.
When Renfield begins to sit down in the chair at Dracula's mansion, he is seen setting his briefcase down, however the briefcase is open and the papers are arranged on the desk in the next shot, without any time for Renfield to have taken them out.
Dracula: Arrogant mortal! You are in my world now and you will never leave this attic alive! I will destroy you, and then I will possess she whom you love the most. And there is not a single thing in the world you can do to stop me!