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!
I think valuable / invaluable actually have different meanings. Something which is invaluable, is so important a value should not be assigned.
Valuable member of society
Invaluable member of society
Neither case leads to a tangible valuation of the member as both have positive meanings. Invaluable is sort of like valuable+1, but both are just invaluable superlatives.
invaluable would be equivalent to priceless.
Priceless and worthless mean the same
Don’t @ me