Another confusing one: inflammable, which means capable of being set on fire but follows the same pattern as other words in which the prefix in- connotes negation, e.g. inflexible.
Further confusing the issue: inflammable means exactly the same thing as flammable.
Ugh, what a country!
Such a nice day. I think I'll go out the window!
Flammable was invented precisely because people assumed "inflammable" meant its opposite. At some point, language consistency becomes less important than random fires being started accidentally.
Apparently that dual usage for the "in-" prefix existed in Latin as well. Inflammo means to burn, but insanus means not healthy.