Program: 5910
Project: 5910
Buggy source:
@Override public void mutate(final ILoggingEvent event_){
  final SLoggingEvent1 event=(SLoggingEvent1)event_;
  final String application=Strings.emptyToNull(Objects.firstNonNull(this.application,DefaultLoggingEventMutator.defaultApplication));
  final String component=Strings.emptyToNull(Objects.firstNonNull(this.component,DefaultLoggingEventMutator.defaultComponent));
  final String node=Strings.emptyToNull(Objects.firstNonNull(this.node,DefaultLoggingEventMutator.defaultNode));
  long sequence;
synchronized (this) {
    sequence=this.sequence;
    this.sequence++;
  }
  if (event.mdcPropertyMap == null)   event.mdcPropertyMap=new HashMap<String,String>(3);
 else   event.mdcPropertyMap=new HashMap<String,String>(event.mdcPropertyMap);
  if ((application != null) && !event.mdcPropertyMap.containsKey(DefaultLoggingEventMutator.defaultApplicationMdcName))   event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultApplicationMdcName,application);
  if ((component != null) && !event.mdcPropertyMap.containsKey(DefaultLoggingEventMutator.defaultComponentMdcName))   event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultComponentMdcName,component);
  if ((node != null) && !event.mdcPropertyMap.containsKey(DefaultLoggingEventMutator.defaultNodeMdcName))   event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultNodeMdcName,node);
  event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultSequenceMdcKey,Long.toString(sequence));
}

Fixed version (ground truth):
@Override public void mutate(final ILoggingEvent event_){
  final SLoggingEvent1 event=(SLoggingEvent1)event_;
  final String application=Strings.emptyToNull(Objects.firstNonNull(this.application,DefaultLoggingEventMutator.defaultApplication));
  final String component=Strings.emptyToNull(Objects.firstNonNull(this.component,DefaultLoggingEventMutator.defaultComponent));
  final String node=Strings.emptyToNull(Objects.firstNonNull(this.node,DefaultLoggingEventMutator.defaultNode));
  long sequence;
synchronized (this) {
    sequence=this.sequence;
    this.sequence++;
  }
  if (event.mdcPropertyMap == null)   event.mdcPropertyMap=new HashMap<String,String>(3);
 else   event.mdcPropertyMap=new HashMap<String,String>(event.mdcPropertyMap);
  if ((application != null) && !event.mdcPropertyMap.containsKey(DefaultLoggingEventMutator.defaultApplicationMdcName))   event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultApplicationMdcName,application);
  if ((component != null) && !event.mdcPropertyMap.containsKey(DefaultLoggingEventMutator.defaultComponentMdcName))   event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultComponentMdcName,component);
  if ((node != null) && !event.mdcPropertyMap.containsKey(DefaultLoggingEventMutator.defaultNodeMdcName))   event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultNodeMdcName,node);
  event.mdcPropertyMap.put(DefaultLoggingEventMutator.defaultSequenceMdcKey,Long.toString(sequence));
  final LinkedList<String> mdcInvalidKeys=new LinkedList<String>();
  for (  final String mdcKey : event.mdcPropertyMap.keySet()) {
    final Object mdcValue=event.mdcPropertyMap.get(mdcKey);
    if (!(mdcValue instanceof String))     mdcInvalidKeys.add(mdcKey);
  }
  for (  final String mdcInvalidKey : mdcInvalidKeys)   event.mdcPropertyMap.remove(mdcInvalidKey);
  if (event.argumentArray != null)   for (int index=0; index < event.argumentArray.length; index++)   if ((event.argumentArray[index] != null) && !(event.argumentArray[index] instanceof Serializable))   event.argumentArray[index]=String.valueOf(event.argumentArray[index]);
}