You can put anything in "real world" contracts (in a 'put words on the paper' sense), but there are things that would be considered illegal if brought to court.

You can put anything in "real world" contracts (in a 'put words on the paper' sense), but there are things that would be considered illegal if brought to court. In a sense there are reasonable defaults. So if you sign without reading a contract that 'you sell your self as a slave to me' this contract would have no power because slavery is illegal.