An investigation into mathematical errors made by 50 sixth-grade children attending an international primary school in Lae, Papua New Guinea, is described. The children were given a battery of tests, with one of the tests, the Monash Assessment of Mathematical Performance (MAMP), being administered twice. Errors made by the children on either, or both, occasions they did the MAMP test were analyzed by means of the Newman interview technique. Attention was focused on errors made on one occasion but not on the other: An error of this type was deemed to have been ”careless” if a child confidently demonstrated, to the interviewer, that he or she knew how to obtain the correct answer to the question and if neither the child nor the interviewer could identify why the original error had been made. Of all the errors made by the children on MAMP questions, 21% were in this category. Analysis revealed that the proportion of careless errors correlated significantly and positively with measures of arithmetical competence, mathematical language, and mathematical confidence but negatively with a measure of misplaced confidence.