Bug Patrol #1. There's a bug in this code snippet. Can you find it?

bug_patrol1.java

private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
private final ExecutorService executor = Executors.newFixedThreadPool(1);
private final Set<String> serialsSet = new HashSet<String>();
private static final int NUM_SECONDS = 3;

// ...

private void scheduleTask(final String serial) {
  // Assume this method is called from a thread in executor.
  if (!this.serialsSet.contains(serial)) {
    this.serialsSet.add(serial);
    Runnable task = new Runnable() {

      /** {@inheritDoc} */
      @Override
      public void run() {
        executor.execute(new Runnable() {

          /** {@inheritDoc} */
          @Override
          public void run() {
            processSerial(serial);
            serialsSet.remove(serial);
          }

        });
      }

    };

    this.scheduler.schedule(task, NUM_SECONDS,
    TimeUnit.SECONDS);
  }
}

private void processSerial(String serial) throws Exception {
  // Do something here that may throw an exception.
}