A jumbo shrimp is a classic example of an oxymoron. It's a large, rather than small, shrimp.
A bittersweet moment is a contradictory combination of bitter and sweet.
A living dead man is a paradoxical state where one is alive, but dead at the same time.
A man is a contradiction to himself, as he is both a rational being and a rationalizing being.
There is no such thing as a 'free' lunch, as it's always free but also always a lunch.