From Strunk & White:
Flammable. An oddity, chiefly useful in saving lives. The common word meaning “combustible” is inflammable. But some people are thrown off by the in- and think inflammable means “not combustible.” For this reason, trucks carrying gasoline or explosives are now marked FLAMMABLE. Unless you are operating such a truck and hence are concerned with the safety of children and illiterates, use inflammable.
Thanks, dear reader, should you ever find me, for being concerned with the safety of children and illiterates.
Just not when you’re attempting to communicate in lettered fashion.