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To develop and train defect prediction models, researchers rely on datasets in which a defect is attributed to an artifact, e.g., a class of a given release. However, the creation of such datasets is far from being perfect. It can happen that a defect is discovered several releases after its introduction: this phenomenon has been called "dormant defects". This means that, if we observe today the status of a class in its current version, it can be considered as defect-free while this is not the case. We call "snoring" the noise consisting of such classes, affected by dormant defects only. We conjecture that the presence of snoring negatively impacts the classifiers' accuracy and their evaluation. Moreover, earlier releases likely contain more snoring classes than older releases, thus, removing the most recent releases from a dataset could reduce the snoring effect and improve the accuracy of classifiers. In this paper we investigate the impact of the snoring noise on classifiers' accuracy and their evaluation, and the effectiveness of a possible countermeasure consisting in removing the last releases of data. We analyze the accuracy of 15 machine learning defect prediction classifiers on data from more than 4,000 bugs and 600 releases of 19 open source projects from the Apache ecosystem. Our results show that, on average across projects: (i) the presence of snoring decreases the recall of defect prediction classifiers; (ii) evaluations affected by snoring are likely unable to identify the best classifiers, and (iii) removing data from recent releases helps to significantly improve the accuracy of the classifiers. On summary, this paper provides insights on how to create a software defect dataset by mitigating the effect of snoring.