[thylacine]

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OK maybe a dumb question but...
If one renders the opponent's choices indifferent, how can he make a mistake?

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Not a dumb question at all. If you play in an unexploitable way, you do tend to equalize the expectation of SOME of your opponent's choices, but not necessarily ALL of them. Mistakes are still possible, except in very simple examples.

Also, your opponent also needs to be careful not to play in an exploitable way.