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!
One that sorta works:
it’s all uphill / downhill from here -> it’s only going to get worse
not really; “it’s all downhill from here” means that the hardest part is behind you, and progress will be easier from here