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!
a bit of a stretch but i can imagine someone saying "You're a natural" and "you're a freak" to express someone being abnormally talented at something on their first try.
"You're a natural at rock climbing. You're a rock climbing freak. A freak at rock climbing."