Everything they say should be one-hundred percent correct all the time. If they read from a prepared speech that has been carefully researched, then the answer is yes. Or they may be asked a very good question that gets them legitimately off subject. Even in these situations, the material presented should be correct. But errors do creep in.
People are human.
I used to be unable to understand how my professors could make computational mistakes at the board. They know the material. After having taught for nearly ten years, I now know how it happens, no matter how knowledgeable, no matter how well prepared. It's easy to miss a minor detail when you are focusing on presenting a larger, more complex idea. Come prepared with note-taking materials and supplies.