A friend just had a nip of Hennessey Cognac confiscated by the TSA in Chicago. They said anything over 70% alcohol was flammable and therefore hazardous. Hennessey is 80 proof, and therefore 40% alcohol.
The word "proof" comes from the 18th Century, when liquor was tested to see if it had been watered down by mixing it with gunpowder and seeing if it would ignite. That only works if the booze is at least 57% alcohol. The fact that the gunpowder ignites when the liquor is 100 proof or more is the proof. In the US this was simplified to 50-50, probably by the same government officials who legislated pi equal to 3.
My friend argued with as many as five TSA officers but to no avail. Rather than miss his flight, he abandoned the minibar plunder to the innumerates.
A few years before, I was checking a heavy bag at the Alaska Airlines counter in Las Vegas. A young ticket agent I hadn't seen before asked me what was in the bag to make it so heavy. I said there were a few bottles of wine. She asked how many. I asked if there was some kind of restriction, as I had never heard of such a thing.
She said, "You can't check anything with 70% alcohol, so if you have five or more bottles of wine that adds up to more than 70%."
"I'm not sure math works that way," I said. "You don't add the percentages in different bottles to get a higher percentage. It's all still the same percentage."
"Over 70% isn't allowed," She said.
I assured her there were fewer than five bottles of wine.