Program: 11622
Project: 11622
Buggy source:
/** 
 * Sends the tab list to all subscribed players.
 */
@Override public void update(){
  PlayOutPlayerListHeaderAndFooter headerAndFooterPacket=new PlayOutPlayerListHeaderAndFooter(this.header,this.footer);
  PlayOutTabListItem.PlayOutTabListItemAddPlayer addPacket=PlayOutTabListItem.addPlayerPacket();
  PlayOutTabListItem.PlayOutTabListItemRemovePlayer removePacket=PlayOutTabListItem.removePlayerPacket();
  PlayOutTabListItem.PlayOutTabListItemUpdateGamemode updateGamemodePacket=PlayOutTabListItem.updateGamemodePacket();
  PlayOutTabListItem.PlayOutTabListItemUpdateLatency updateLatencyPacket=PlayOutTabListItem.updateLatencyPacket();
  PlayOutTabListItem.PlayOutTabListItemUpdateDisplayName updateNamePacket=PlayOutTabListItem.updatePlayerPacket();
  Map<UUID,TabListElement> lastSeen=new LinkedHashMap<>();
  Map<UUID,TabListElement> currentElements=new LinkedHashMap<>();
  this.lastSeen.forEach(e -> lastSeen.put(e.getUuid(),e));
synchronized (this.lock) {
    this.elements.forEach(e -> currentElements.put(e.getUuid(),e));
  }
  if (currentElements.containsKey(null)) {
    throw new IllegalStateException("String_Node_Str" + currentElements.get(null) + "String_Node_Str");
  }
  lastSeen.entrySet().stream().filter(e -> !currentElements.containsKey(e.getKey())).forEach(e -> {
    removePacket.removePlayer(e.getKey());
    currentElements.remove(e.getKey());
  }
);
  currentElements.forEach((key,value) -> {
    if (lastSeen.containsKey(key)) {
      TabListElement last=lastSeen.get(key);
      if (!Objects.equals(value.getDisplayName(),last.getDisplayName())) {
        updateNamePacket.update(key,value.getDisplayName());
      }
      if (value.getGameMode() != last.getGameMode()) {
        updateGamemodePacket.update(key,value.getGameMode());
      }
      if (value.getPing() != last.getPing()) {
        updateLatencyPacket.update(key,value.getPing());
      }
    }
 else {
      addPacket.addPlayer(value);
    }
  }
);
synchronized (this.lock) {
    this.lastSeen.clear();
    this.lastSeen.addAll(this.elements);
  }
  this.users.forEach(p -> {
    TridentPlayer player=(TridentPlayer)p;
    if (removePacket.getActionCount() > 0)     player.net().sendPacket(removePacket);
    if (addPacket.getActionCount() > 0)     player.net().sendPacket(addPacket);
    if (updateGamemodePacket.getActionCount() > 0)     player.net().sendPacket(updateGamemodePacket);
    if (updateLatencyPacket.getActionCount() > 0)     player.net().sendPacket(updateLatencyPacket);
    if (updateNamePacket.getActionCount() > 0)     player.net().sendPacket(updateNamePacket);
    player.net().sendPacket(headerAndFooterPacket);
  }
);
}

Fixed version (ground truth):
/** 
 * Sends the tab list to all subscribed players.
 */
@Override public void update(){
  PlayOutPlayerListHeaderAndFooter headerAndFooterPacket=new PlayOutPlayerListHeaderAndFooter(this.header,this.footer);
  PlayOutTabListItem.AddPlayer addPacket=PlayOutTabListItem.addPlayerPacket();
  PlayOutTabListItem.RemovePlayer removePacket=PlayOutTabListItem.removePlayerPacket();
  PlayOutTabListItem.UpdateGameMode updateGamemodePacket=PlayOutTabListItem.updateGamemodePacket();
  PlayOutTabListItem.UpdateLatency updateLatencyPacket=PlayOutTabListItem.updateLatencyPacket();
  PlayOutTabListItem.UpdateDisplayName updateNamePacket=PlayOutTabListItem.updatePlayerPacket();
  Map<UUID,TabListElement> lastSeen=new LinkedHashMap<>();
  Map<UUID,TabListElement> currentElements=new LinkedHashMap<>();
  this.lastSeen.forEach(e -> lastSeen.put(e.getUuid(),e));
synchronized (this.lock) {
    this.elements.forEach(e -> currentElements.put(e.getUuid(),e));
  }
  if (currentElements.containsKey(null)) {
    throw new IllegalStateException("String_Node_Str" + currentElements.get(null) + "String_Node_Str");
  }
  lastSeen.entrySet().stream().filter(e -> !currentElements.containsKey(e.getKey())).forEach(e -> {
    removePacket.removePlayer(e.getKey());
    currentElements.remove(e.getKey());
  }
);
  currentElements.forEach((key,value) -> {
    if (lastSeen.containsKey(key)) {
      TabListElement last=lastSeen.get(key);
      if (!Objects.equals(value.getDisplayName(),last.getDisplayName())) {
        updateNamePacket.update(key,value.getDisplayName());
      }
      if (value.getGameMode() != last.getGameMode()) {
        updateGamemodePacket.update(key,value.getGameMode());
      }
      if (value.getPing() != last.getPing()) {
        updateLatencyPacket.update(key,value.getPing());
      }
    }
 else {
      addPacket.addPlayer(value);
    }
  }
);
synchronized (this.lock) {
    this.lastSeen.clear();
    this.lastSeen.addAll(this.elements);
  }
  this.users.forEach(p -> {
    TridentPlayer player=(TridentPlayer)p;
    if (removePacket.getActionCount() > 0)     player.net().sendPacket(removePacket);
    if (addPacket.getActionCount() > 0)     player.net().sendPacket(addPacket);
    if (updateGamemodePacket.getActionCount() > 0)     player.net().sendPacket(updateGamemodePacket);
    if (updateLatencyPacket.getActionCount() > 0)     player.net().sendPacket(updateLatencyPacket);
    if (updateNamePacket.getActionCount() > 0)     player.net().sendPacket(updateNamePacket);
    player.net().sendPacket(headerAndFooterPacket);
  }
);
}