Factual error: In the episode where Sydney is supposed to be in Helsinki, there are visible skyscrapers in the background. There are no skyscrapers in Helsinki.
Day 4: 2:00 P.M.-3:00 P.M. - S4-E8
Factual error: When Edgar proves that Mary-Ann Taylor framed Sarah to be the rat in CTU, he claims that she remotely took control of her computer by sending the signal over "a plain old AC power line." This is not possible, unless the computers were equipped with special hardware to separate the AC from the signal. If they were, this traffic would have been observed by James Heller's assistant.
Factual error: When the cops need to arrest a medical professional, they are frequently shown barging into his/her office, exam room, or even operating room. This would never happen in real life and is strictly forbidden, as it is a gross violation of the patient's privacy, and in the case of the OR, could contaminate the sterile environment, thereby jeopardizing patient safety as well.
There's No Place Like Home - S2-E1
Factual error: This is at the end of the episode, when Jordan takes the key that she found for the 66 Ford and tries it in the ignition it should have been a square key. Round keys were for the doors and trunk. Back then they made two keys - one that opened the car doors and trunk and one that started the ignition. She never even tried the key in the trunk.