Are ANY and ALL contracts valid?

I ponder this due to, once again, "Bubble Theory".

If a person thinks they have no choice but to agree to something that they know isn't right and that will harm them, is their agreement really consensual? Can a property owner make any demands they want and expect compliance? Can you really consent to being attacked or abdicating (or negotiating away) your rights?

I see parallels in the debate over "intellectual property" and in a post I read about sweatshops.