Abstract: The idea of margin for error of principles is used to analyse the Surprise Examination Paradox. Although the pupils in the story know that there will be a surprise examination, they do not know that they will know tomorrow that they will know the day after that ..., because each iteration requires a further margin for error until nothing is left. A similar account is suggested for knowledge of rationality in Iterated Prisoner's Dilemma. Versions of the Surprise Examination Paradox in which the pupils know only that there will not be an expected examination are used to criticize alternative accounts of the paradox.

Keywords: error, iterationmargin, Prisoner's Dilemma, rationality, Surprise Examination Paradox,