Factual error: Calleigh Duquesne frequently wears completely inappropriate clothing throughout the whole series. She often wears blouses that are so low cut that the neckline is below the lower curve of her breasts. No officer of the court in the US would be allowed to dress this way. There is no grey area here, and this is not a character mistake - first time a CSI turned up at work dressed like that, she'd be sent home to change. Second time, she'd be on suspension until she agreed to change her dress standards. Also (and this regularly happens in CSI: NY as well) she is regularly wearing high heels - also a certain no-no for as CSI. Incidentally, male CSIs have equally strict dress standards.
Factual error: Every time the investigators deal with IP-addresses, the addresses on display are impossible. Each of the four parts of an IP-address has to be between 0 and 255. This isn't equivalent to the movie-specific 555 area code for phone numbers - having an IP address outside that range is like having a phone number which includes the % symbol - it's just impossible.
Revealing mistake: Almost all fingerprints throughout the whole show match by 99.32 percent. I have personally tracked it through series 6, 7, 8 and 9 and almost no fingerprint checks resulted in a different match. (Whether this has started in earlier series has to be confirmed).
Other mistake: Every time they use Nikon DSLR you see how it looks when you look inside camera's viewfinder. And it's wrong. Nikon viewfinders don't have 9 focus points (I've checked at www.dpreview.com). Almost every time they shoot photos ranging from 13 to 15. Biggest number they shot was 17. Sometimes they shoot 13, 14 and then goes 13 again. Numbers are wrong also because it should represent how many shots are left to take, not how many you have shot.
Factual error: During many of the episodes, we often see an overhead shot one of the CSI agents in the Hummer speeding down the Miami (or Los Angeles, where it is filmed) highway systems to a crime scene or what not. If you pay close attention, there is almost never any other traffic on the highways besides them. Very unlikely if not impossible, especially in Miami (or Los Angeles).
Factual error: CSIs in real life are not police detectives, nor are they beat cops. Crime Scene Investigators are just that: they're regular people, some with 4-year-bachelor's° in STEM fields, others completing Criminology Associate's degrees, who are ONLY responsible for investigating and gathering evidence at crime scenes and then serving as expert witnesses in court. In fact, they spend more time in court testifying of the evidence they collected than anywhere else. They don't interrogate suspects, nor do they chase them down and arrest them.