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. This is commonly known as "the CSI effect".
CSIs are not detectives. They are civilian employees. 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.
In many occasions, when arresting a suspect, the team is able to exact a confession but never the Miranda's right are read to the suspect, which can and will be used to render the confession unusable in a court of law.
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 simply not possible in the real world. This is commonly known as "the CSI effect".
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.
In the opening credits of Season 1, there is a quick glimpse of someone (probably Warrick Brown) playing blackjack. Three sets of cards are visible - the one in the player's hand, scratching the table (calling for a hit) and two hands immediately to that player's left. Because blackjack is played from the dealer's left to right, the two hands to the player's left have not yet been played, yet the cards are face-down between their respective bets and the dealer. In a real game, the cards would have been dealt on the far side of the chips (toward the player). If the hands were in play, the cards would either be in the player's hand or under the chips (indicating a stand). As it is, the cards would never be in the position they are in. (Even if Warrick were playing all three of the hands, they would be dealt in alternating location - one in front of the chips and the next behind, never two consecutive hands in front of the chips.) Also, one of the hands is laid on the table such that it is partially covering its bet - something the casino would not allow because of the constant surveillance of the players' bets.
In many episodes the CSI's are shown taking photos from crime scenes, then enlarging and enhancing them to get clues that let them solve that weeks crime. But in many instances, the original photo is blurry and out of focus and no matter how much you enlarge and enhance it you would not get the clear image of the clue that they always get on CSI.