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An attacker can only try to change one of his own transactions to take back money he recently spent. The race between the honest chain and an attacker chain can be characterized as a Binomial Random Walk. Nodes are not going to accept an invalid transaction as payment, and honest nodes will never accept a block containing them. Even if this is accomplished, it does not throw the system open to arbitrary changes, such as creating value out of thin air or taking money that never belonged to the attacker. The probability of an attacker catching up from a given deficit is analogous to a Gambler's Ruin problem.