Factual errors: While often the science and technology portrayed in the series (as well as the other CSI series) is accurate or mirrors sound scientific principle, there have been times when methods have led to results that are simply not possible in the real world.
Revealing mistakes: Throughout the series it is clear that the show is not filmed in New York as the same set is used for all the streets.
Continuity: In the episodes where the blood is spattered or in a large pool, the pattern of the blood varies from scene to scene. For example: A man is killed and the blood is sprayed across a desk. The pattern in scene 1 (finding the body) will be different than in scene 2 (going back to the crime scene for further evidence).