code: public List<FailedEventInvocation> getFailedInvocations() {
          synchronized (this.mutex) {
              if (this.failedEvents == null) {
                  return Collections.emptyList();
              }
              return Collections.unmodifiableList(this.failedEvents);
          }
      }
docs: Gets the list of failed invocations that has been collected by this collector.
  
  @return The failed invocations.