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!
People will defend those until they’re blue in the face, and I don’t know why. It’s always the “language is always in a state of flux” bullshit, like improper negations are normal and expected.