When Rainsford and Zaroff are exiting the bedroom in a scene 14 minutes into the movie, Rainsford is quite clearly entering the door first. When we cut to the hallway, it is Zaroff who is leading the way.
The island is described by Rainsford as "small as a deer park", but it contains a dramatic waterfall. Such a fall would have to have been fed by a large lake on a much larger island to flow at such a high volume.
A few seconds after Bob and Eve first enter the jungle together, the branches and leaves of a small tree in the foreground shake and sway dramatically. Whilst the tree is swaying, presumably from the wind, the rest of the vegetation in the scene is absolutely stationary.