With inductive logic(based on observation) there is always room to be wrong. You can never be 100% certain of anything external to your own mind. Also, you might be operating on a heuristic. A heuristic is a short cut way of thinking. For instance, that guy has a lot of cars come and go to his house, me must be selling drugs, because the last guy who lived there had cars come and go. In reality, there could be many explanations for why so many cars are coming to and fro. It could just be coincidence.