When Little Kong fights the Nothosaurus in the cavern following the discovery of the treasure they are both reflected in the glass used in the process shot superimposed on Denham and the girl in the background.
While singing "Runaway Blues" in the Monkey Show, it's clear that Hilda (as La Belle Helene) isn't really playing her guitar. Her finger movements in no way match the chords she is strumming.