For example, “flammable” and “inflammable” both describe an object that can easily catch on fire. I can also think of “ceased” and “deceased”, both of which can mean someone or something has been brought to an end.
edit: Some people are including words that can also mean its opposite (like sanction or table), those are cool too! The more weird words, the better!
Yeah, adding in- as a prefix often does mean that, but it can also mean in, on, or into (among others) as a prefix or just part of the core word. And in this case inflammable comes from adding the suffix -able to inflame, a word that already starts with in- in it’s into meaning. And one definition of inflame is “to burst into flames.”