Plot hole: A large part of Nick's plotline revolves around the discovery that the couple's stolen ring has a fake diamond switched out for the real one. But the ring was left as collateral for a very expensive necklace for the woman to wear that night. There is no way that a jewelery store would accept a ring for collateral without appraising the ring's/stone's value first. And in that inspection, they would have discovered the switch and not accepted it.
Plot hole: Grissom examines the victim's body and immediately rules it a homicide because he was wearing eyeglasses. He states that suicide is a cowardly act and no coward wants to see their death and would have removed their glasses before committing suicide. What a completely unfounded, and unscientific, statement. Suicide being an act of cowardice is his opinion and not a scientific fact and they don't work off opinions: they always state how they work off the evidence.
Plot hole: Lizzie had the car towed to icebox canyon. When you see her watching the car being lowered onto Sara, there is no tow truck there, there is no sound of any machinery lowering the car, and when the shot pans back there is nothing there that would have helped her move the car. She is not strong enough to hold up a car, put an unconscious person under it while holding it up, and then lower it without some kind of winch.
Plot hole: A head without a body is found early in the show. Then, a body that has been decapitated, skinned, and severed of hands and feet is found. The CSI team assume the two are related until the coroner states that the body isn't even human. He also mentions that he has no idea what kind of animal it is and they'll need to consult an anthropologist. Everyone is shocked to learn the body is that of a gorilla. The body is humanoid (2 arms, 2 legs). It is larger than human size. The only thing it can possibly be is a gorilla. It must be a primate for having a humanoid shape and must be a gorilla because that is the only primate larger than man. There was no need to bring in an anthropologist (which should have been a zoologist if they truly had NO idea what kind of animal it was). It should have been painfully obvious to the CSI team, who are experts in all fields, that the body was a gorilla.
Plot hole: How did the occupant of the room where the dry ice was placed only burn his index finger when he picked it up? His thumb, other fingers, and palm were all undamaged. Was he wearing special gloves that had only the index finger missing when he picked up the dry ice?
Plot hole: Two women die after a medical error occurs after undergoing an "anti-aging" medical procedure. A 30% solution of hydrogen peroxide is given intravenously instead of a 3% solution. According the the Material Safety Data Sheets (MSDS) a 30% solution is extremely corrosive and will blister skin on contact. 1) How did the women not bolt upright screaming as this extremely corrosive solution was pumped throughout their veins. 2) How did they not die in the clinic instead of the hours after as depicted in the show? Even in the CSI crew little experiment, the red blood cells immediately burst when exposed to the 30% solution.
Plot hole: While searching through the dead computer guy's CDs Nick finds a CD supposedly burned that night, containing data gathered at the time of death, but by the time the CD was burned the guy would have already been dead, so who took it out of the computer and filed it away?
Plot hole: The bodies were pink because of exposure to the lower temperatures created by the dry ice venting into the room. None of the CSI team noticed a chill when they entered the room? None of them noticed the bodies were cold to the touch? No one noticed that the body or liver temperatures didn't match with the estimated time of death?
Plot hole: Series 9 episode 12: "Agent Hartford" had no fingerprints: the FBI "removed" them. But when they find the street piece that Vinny had, they dust it and say that Hartford's prints were all over it. This might be OK, if he were a real agent on the books (but they never heard of him at Quantico), or if he were a real criminal - but in that case, they would have sussed it all out earlier.
Plot hole: Episode 9-18, "Turn, Turn Turn": Greg states that the dead motel manager crawled up into the ceiling the same way that he did, and you see Nick open the access panel in the ceiling. The manager couldn't have closed the ceiling panel after him and no one was managing the hotel in the week he was missing. The panel should have been open when Nick came into the room. The new manager stated he didn't know if there was a panel in that room, since he just got there.
Plot hole: A central plot device in this episode is that there is no six letter word made up of the letters EXVIN, so the murdered man cheats at the word game by playing a word he knew to be inadmissible - exvin, a wine connoisseur who no longer drinks. Since he is supposed to be a stone cold killer player at this word game, don't you think he would have thought of Vixen? Sara Sidle points that word out later - why wouldn't a world champion word game player have figured it out, using a safe, common word and avoiding a possible challenge?