Ducky was born and raised in Scotland and has a distinct Scottish accent, however "Young Ducky" is extremely English.
After showing a very large back tattoo of a cross while she's changing clothes in front of McGee, various outfits of Abby's have in succeeding instances revealed her to not have said tattoo.
Between 2003 and 2005, "NCIS" and JAG Justicia Militar (1995) had different Secretaries of the Navy, even though the two shows exist within the same universe.
When analyzing shell casings, there's more than one reference about the fact that it "could be either a Glock or a Beretta". The difference would be readily apparent to an experienced observer. Glock: the firing pin leaves on the primer a distinctive rectangular mark plus, in many cases, the extraction leaves a mark near the mouth of the case. Beretta: they leave just a small mark on the primer left by the percussion pin. There's no way you can mistake a Glock and a Beretta, given a spent casing fired from either pistol.
When the series began, Gibbs was 53 years old. As of 2018, Gibbs is at least 8 years beyond NCIS field agent mandatory retirement age of 57.
Gibbs's 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.
While often the science and technology portrayed in this series (as well as the CSI series, which is also shown on CBS) is accurate or mirrors sound scientific principle, there have been times when methods have led to results simply not possible in the real world.
When ever a handgun is fired, the muzzle remains level and steady. Even in the hands of the most experienced shooter, there is recoil and slight muzzle rise, or "bounce", especially when firing multiple shots in quick succession. This never happens to the agents.
Throughout the series, it is obvious the elevator (lift) at the NCIS offices only has one set of doors. In reality, passenger elevators have doors on both the elevator and the elevator shaft, and both would be visible when the doors open and close.
The computers for the characters are always left on and unlocked, showing various screens of data. No company and certainly no law enforcement agency would allow people to walk away with their computers unlocked.
In (Kill Ari (Part II) (2005)) the license plate on Ari's SUV is shown as a Virginia tag# 724 TJA, but in (Model Behavior (2005)), the same tag is shown on the medical examiner's truck. Additionally, since the ME truck is an official federal government vehicle, it would have US Government, not state, tags.
In the episode "Hiatus Part 2" it is revealed that Gibbs joined NIS (changed the same year to NCIS) in 1992 following the death of his wife and daughter. However, in the JAG episode "Ice Queen" in 2003 he tells Lt.Cmdr. Rabb that he has been an agent for 19 years, which would mean he would have joined NIS in 1984.
When the scene is towards the team's office windows, a view of the Display Ship Barry, The Anacostia River, and Anacostia can all be seen, and is correct. However at the top of the view is the Rotunda of the US Capitol Building, which is actually north of the Navy Yard and not south like all the other objects seen through the window.
One team of investigators secures a crime scene. Another team interviews suspects and witnesses. Another team, usually a local medical examiner's office, performs autopsies. Another team, often under contract with an agency, analyzes evidence. In the NCIS universe, all of those functions are conveniently handled by 8-10 people in almost every case.
Numerous times, autopsies are conducted by Ducky, who is wearing the same clothes he wore in the field. Not only does this promote cross-contamination, but it is also highly unsanitary as well as an OSHA violation.
Many times throughout the series, forensic weapons specialists, assassins, and a retired Marine Gunnery Sergeant refer to a "suppressor" as a "silencer". While the audience of the show may know it as such, the characters should not.
Abby tells people that whenever she finds something important to the case they are working on, Gibbs knows and just shows up in her lab. However, there are multiple times throughout her run on the show that she calls Gibbs and tells him she found something.