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!
deceased specifically refers to death though. ceased mean something stopped in its track. " no offense and offensive" means the same thing if the context is describing someone or something. shelled/deshelled both means shells are remove from a plant product.