Indispensable / Indisputable
Indispensable means absolutely necessary; indisputable means factual, without a doubt, and not arguable.
e.g. Air is indispensable to life.
e.g. It is indisputable that the verdict of the judge is final.
Prepossessing / Preposterous
Prepossessing means attractive or impressive; preposterous means absurd or contrary to reason.
e.g. She had put on a prepossessing dress to impress the audience.
e.g. You look preposterous in that ridiculous outfit!
Irritable / Irritant
Irritable means easily made angry; irritant means causing anger or discomfort.
e.g. He has a short temper and is easily irritable.
e.g. Nobody likes him because of his irritant behavior.
Preparation / Preparedness
Preparation means getting ready; preparedness is a state of being prepared.
e.g. The country's preparations for war are complete.
e.g. The country is in preparedness for war.
Inflammable / Inflammatory
Inflammable means easy to catch fire; inflammatory means causing unrest or bad feelings.
e.g. Be careful! This kind of material is inflammable.
e.g. The man's speech was not only anti-government but also inflammatory,