A tale unfolds around a persistent NoClassDefFoundError encountered during the execution of a Flink job, leading to an exploration of JVM class loading and dynamic tracing. The error, which stemmed from a Redis client library integrated into a Flink job, was perplexing because the necessary class was confirmed to be in the JAR file. Delving deeper, the issue was traced back to the URLClassLoader being closed prematurely, preventing the class from being loaded when needed. By utilizing BTrace, a dynamic tracing tool, the team identified that the class loader was being closed before the class could be loaded, due to an Akka-based ActorSystem continuing to process Redis messages independently of the application lifecycle. The solution involved aligning the Redis client shutdown with the Flink application lifecycle using a RichSinkFunction, thus preventing the class loader from being closed prematurely and resolving the NoClassDefFoundError. This debugging journey not only resolved the issue but also enhanced the team's understanding and provided them with a new tool, BTrace, for future debugging scenarios.