Reliability of build outcomes is a cornerstone of effective Continuous Integration (CI). Yet in practice, developers often struggle with non-deterministic issues in the code or CI infrastructure, which undermine trust in build results. When faced with such unexpected outcomes, developers often repeatedly rerun jobs hoping for true success, but this practice is known to increase CI costs and reduce productivity. While recent studies have focused on intermittent job failures, no prior work has investigated silent failures, where build jobs are marked as successful but fail to complete all or part of their tasks. Such silent failures often go unnoticed, creating an illusion of success with detrimental consequences such as bugs escaping into production. This paper presents the first empirical study of silent failures through the practice of rerunning successful jobs. An analysis of 142,387 jobs across 81 industrial projects shows that 11% of successful jobs are rerun, with 35% of these reruns occurring after more than 24 hours. Using mixed-effects models on 32 independent variables (AUC of 85%), we identified key factors associated with reruns of successful jobs, notably testing and static analysis tasks, scripting languages like Shell, and developers prior rerun tendencies. A further analysis of 92 public issues revealed 11 categories of silent failures aligning with these factors, the most frequent being artifact operation errors, caching errors, and ignored exit codes. Overall, our findings provide valuable insights into the circumstances and causes of silent failures to raise awareness among teams, and present solutions to improve CI reliability.

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