Other mistake: In the scene where Renfield enters Dracula's castle, it is past midnight and on the outside, it is dark. But there is a LOT of bright light streaming through the windows into the castle, throwing well-defined shadows. (00:09:10)
Other mistake: A crewman tells the ship's captain he knows there are four stowaways because he heard them singing "Sweet Adeline," a popular barbershop quartet song. Harpo never spoke in any of the Marx Brothers films. The crewman could not have heard four voices.