[JocK]

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Playing optimally does not offer your opponents the opportunity to make mistakes (except for dominated decisions which are always availble no matter how you play).

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Although this might be true for games like Roshambo, this is not correct in general (but it depends a bit on what you mean by 'dominated decisions').

In more complex games strategies do exist that can lead to gains against certain of opponent's choices (i.e. these are non-dominated strategies), but the same strategies are dominated by the Nash strategies. Selecting such a strategy that is dominated by the opponent's Nash strategy would be a mistake.

In general it is true that Nash-strategies are not maximally exploitive towards opponent's errors, but they do exploit some of these errors.