(1) "Whatever arises from a just situation by just steps is itself just."
As Cohen explains it:
A just situation, here, is one in which everyone has all and only those holdings which they ought to have, and 'just steps', for Nozick, are human actions that are free of injustice, in the sense that no-one behaves with force or fraud in the course of them. (Self-Ownership, Freedom, and Equality, p.39.)
But as it stands, (1) is plainly false - even without assuming a patterned conception of justice - for unjust situations can arise through accident or ignorance. Suppose I cheaply sell you a diamond I justly own, which we both think is glass. If you later discover its true value, and yet keep it for yourself, "few would think that justice is fully served... even though no one behaved unjustly in the generating transaction." (Cohen, p.45) Or, if you're inclined to blame people for their ignorance, suppose we were comparing stones, and they got mixed up. You took away my diamond, and I was left with your glass stone. It seems an injustice has occurred - we no longer possess our rightful holdings - even though no-one has committed any wrongdoing.
Suppose we refine (1) so as to exclude accident or error. This would avoid the above counterexamples, but it would also leave it politically impotent. Ignorance and accidents are common in real life, so (1) would no longer justify free-market capitalism. Voluntary exchange is no guarantee that people are getting what they deserve.