In World War II, every country was looking for an edge, so it's pretty amazing that the Nazis found one and then decided against it – and rightly so. Chlorine trifluoride ignites on contact with almost any substance, burns at over 2000°C, and will melt tanks, bunkers, schools, and pretty much anything it comes into contact with.
Some things are better left alone.
It must have been one helluva weapon if even Hitler didn't use it (Spoiler Alert: It was).
In 1930, German scientists came across a volatile new discovery. Dubbed "Substance N," the concoction boiled at room temperature and produced a toxic gas. When ignited, this toxic gas also burned at thousands of degrees Celsius. After decomposing, it turned into the slightly-less-dangerous-hydrochloric acid (that was actually more dangerous because it occurred as steam). It was also corrosive and exploded on contact with water. Or carbon, which is everywhere. This stuff set fire to asbestos.
At first glance, it might seem like an ideal weapon of war, one that keeps killing in many, many forms and doesn't stop. And the Nazis thought so too. For years they tried to produce enough of the material to effectively weaponize it. The stuff ate through everything, and what it didn't eat through, it burned.
It burns concrete. No joke.
Nazi Germany would have totally used this weapon if they could have produced and stored enough of it to actually convert to weapons. If they could have safely transported those weapons and used them before the chemical violently exploded, burned, or otherwise ate through whatever it was in.
Turns out the only safe way to store it is to seal it in containers made of steel, iron, nickel, or copper after they've been treated with fluorine gas. The fluorine protects the other substances from the Chlorine Trifluoride. The stuff is so unstable, Chemist John D. Clark once said the best way to deal with a failure to contain the resulting fire from a chlorine trifluoride storage failure is "a good pair of running shoes."