Plot hole: When Miss Price, Professor Browne and the children return from their magical journey to the island of Naboombu and realize they no longer have the Star of Astroroth pendant, Miss Price says, "I should have known it would be quite impossible to take an object from one world into another." Yet, they had no problem taking themselves, the bed, blankets and pillows from one world into another.
Plot hole: Major plot leak: How could Flint's ship get into the lake atop a tall volcano all in one piece? If he wanted to merely hide the treasure in order to recover it later, the whole setup, including the lake's draining system, would be totally unnecessary, and it looks all too deliberate for the ship to be sitting in the lake by accident (say, thrown in by a violent storm).