When Dracula's brides converge on Renfield after he has passed out, Dracula enters and motions them away. As they are walking backwards, one bride steps on another bride's dress causing one bride to "catch" another. It is possible that she may have stepped on her own dress.
Dr. Seward threatens to have Renfield confined in a "strait-jacket". Although this is probably a translation convention for the American audience rather than an unintentional slip, a British doctor would say "strait-waistcoat," which is what he says in Bram Stoker's novel and other filmed enactions of this scene.
When the street doors of the London concert hall open to admit Dracula, the orchestra can be heard playing Franz Schubert's "Unfinished Symphony". But in the next shot, an instant later, they are playing the conclusion of the prelude to Richard Wagner's "Die Meistersinger".
Joan Standing is wrongly credited as a maid, but she was actually playing the part of Briggs (a nurse). Moon Carroll played the uncredited part as a maid. She's the one who faints when Renfield is laughing very scary.