This is not a slip up. "Inflammable" and "flammable" mean exactly the same thing. Namely, they both mean "burns easily." The word "flammable" actually did not start getting used on warning labels until the 1920s, due to the fact that previous labels of "inflamable" caused confusion for the general public. So, during the Civil War, when this movie takes place, the "inflammable" label is accurate.
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