Ziva smashes Sgt. Barnes' face into a plate of food as she handcuffs him but after there is no trace of food on his face.
In addition to liquid nitrogen disappearing quickly without a trace, it boils even in a freezer, caused by the difference in temperature, not contact with organic material. No one could confuse this clear, colder than ice, violently bubbling liquid for hot chocolate, not to mention the pressure that would build up in a simple thermos.
When a blow-up picture of Pvt Singer's Military ID card is displayed, his enlisted pay-grade of "E-1" is clearly posted with his rank "Pvt". For E3 and below, the pay-grade is never displayed. His ID card should have read "PVT/LCPL" with no subsequent pay-grade of E1, E2, or E3.
Abby calls Gibbs back to the monitor as she's found something in one of the scans; the scene then shifts to show the gaseous makeup of the sample (Turpentine, Nitrogen, Acetone, and Benzine). Benzine isn't C6H6, benzene is; benzine is also a petroleum fragment and rather hard to handle outside of a laboratory setting as its boiling point is somewhere between 42 and 62 degrees Centigrade (107.6-143.6 degrees Fahrenheit) and its flash point (the temperature at which it can be set on fire with assistance) is less than 0 degrees Centigrade (32 degrees Fahrenheit).
It is extremely unlikely for a life insurance company to cover combat-related death of active military personnel especially hundreds of thousands of dollars' worth of coverage.
First Abby, then McGee are guilty of using the Enhance Button trope: they enlarge and "enhance" bad quality video footage to reveal legible license plates. This means changing the focus of recorded footage to conjure up information that simply doesn't exist in said footage.
Access to the murder weapon is all but forgotten in the investigation.
Liquid nitrogen is not easy to acquire, but it is never explained how the killer, or any of the suspects, got hold of it.