In Peter Jackson's version of The Lord of the Rings, Isengard is something of a flat valley with the tower Orthanc at its center. All around the tower, in a three-dimensional mandala of evil, are the charnel pits and Uruk-hai spawning chambers—the machinery with which the twisted Saruman the White is generating an army.
In "The Fellowship of the Ring," Jackson shows Gandalf riding into Isengard. Saruman meets Gandalf outside; the scene is cut, and we see both wizards inside Orthanc. As the tenor of their conversation grows more sinister, Gandalf realizes that Saruman has changed allegiance and is now a minion of the Dark Lord, Sauron.
So the question is this: how did Gandalf, as he was riding toward Orthanc, not realize Saruman had turned? Surely he would have seen several square miles of fumes and smelled the intense charnel stench, no? I don't recall clearly, but I think Tolkien's novel actually elides all of this, leaving it to the reader to imagine how Gandalf's encounter with Saruman went. Jackson's movie, by contrast, produces what may be an unforced error: Gandalf's inability to see what Saruman is doing on Isengard's property means either that Saruman is very good at hiding his operation (and we're given no hint that he's hiding anything), or that Galdalf, for all his years and wisdom, is unwontedly imperceptive.