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.
The investigation team routinely photographs the crime scene before allowing evidence to be touched or moved. Many times, however, a piece of evidence is picked up and handled before being photographed.
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).
Throughout the series, probably to avoid copyright infringement issues, incorrect references are made to the New York Subway system. On subway entrances, the 1, 2, and 3 lines are shown in yellow, when they are actually red. A character blames his lateness on the fact that the Q train was changed to an express, whereas in real life the Q is normally an express.
Numerous times, autopsies are conducted by people who were wearing the same clothes they wore in the field. Not only does this promote cross-contamination, but it is also highly unsanitary as well as an OSHA violation.
Although the CSIs are also detectives, this is exceedingly rare in actual life. It is considered an inappropriate and improbable practice to allow CSI personnel to be involved in detective work as it would compromise the impartiality of scientific evidence and would be impracticably time-consuming.