Inflammable.
The problem is that the prefix in- has two origins. One is Germanic, and intensifies the meaning of the compound; the other derives from a Latin prefix in-, which negatives the meaning of the compound. Normally, people know which is which by context.
AFAIK, the original word was "inflammable" with intensifying in-. But, of course, some people took the wrong meaning, so the word "flammable" was invented, for labelling purposes. For me, the word is "inflammable" in normal usage (as in "highly inflammable" or "an inflammable personality"), but I'd for sure put "flammable" on a bottle of solvent.
P.S., the true pedants will notice that I have somewhat simplified the in- thing; and also that "flammable" did exist in the 1800s, but was rare until its adoption on elf and safety grounds.