Program: 27608
Project: 27608
Buggy source:
@Override public Optional<World> loadWorld(String worldName){
  final Optional<World> optExisting=getWorld(worldName);
  if (optExisting.isPresent()) {
    return optExisting;
  }
  File file=new File(getFolderName(),worldName);
  if ((file.exists()) && (!file.isDirectory())) {
    throw new IllegalArgumentException("String_Node_Str" + worldName + "String_Node_Str");
  }
  AnvilSaveHandler savehandler=new AnvilSaveHandler(new File(getFolderName()),worldName,true);
  int dim=0;
  WorldInfo worldInfo=savehandler.loadWorldInfo();
  if (worldInfo != null) {
    if (!((WorldProperties)worldInfo).isEnabled()) {
      SpongeMod.instance.getLogger().error("String_Node_Str" + worldName + "String_Node_Str");
      return Optional.absent();
    }
    if (!SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId()).isPresent()) {
      SpongeMod.instance.getSpongeRegistry().registerWorldProperties((WorldProperties)worldInfo);
    }
 else {
      worldInfo=(WorldInfo)SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId()).get();
    }
    dim=((IMixinWorldInfo)worldInfo).getDimensionId();
  }
 else {
    return Optional.absent();
  }
  WorldSettings settings=new WorldSettings(worldInfo);
  if (!DimensionManager.isDimensionRegistered(dim)) {
    DimensionManager.registerDimension(dim,((SpongeDimensionType)((WorldProperties)worldInfo).getDimensionType()).getDimensionTypeId());
  }
  WorldServer world=(WorldServer)new WorldServer((MinecraftServer)(Object)this,savehandler,worldInfo,dim,this.theProfiler).init();
  world.initialize(settings);
  world.provider.setDimension(dim);
  world.addWorldAccess(new WorldManager((MinecraftServer)(Object)this,world));
  MinecraftForge.EVENT_BUS.post(new WorldEvent.Load(world));
  if (!isSinglePlayer()) {
    world.getWorldInfo().setGameType(getGameType());
  }
  this.setDifficultyForAllWorlds(this.getDifficulty());
  if (((WorldProperties)worldInfo).doesKeepSpawnLoaded()) {
    this.prepareSpawnArea(world);
  }
  return Optional.of((World)world);
}

Fixed version (ground truth):
@Override public Optional<World> loadWorld(String worldName){
  final Optional<World> optExisting=getWorld(worldName);
  if (optExisting.isPresent()) {
    return optExisting;
  }
  File file=new File(getFolderName(),worldName);
  if ((file.exists()) && (!file.isDirectory())) {
    throw new IllegalArgumentException("String_Node_Str" + worldName + "String_Node_Str");
  }
  AnvilSaveHandler savehandler=null;
  if (FMLCommonHandler.instance().getSide() == Side.CLIENT) {
    savehandler=new AnvilSaveHandler(new File(FMLCommonHandler.instance().getSavesDirectory() + File.separator + getFolderName()),worldName,true);
  }
 else {
    savehandler=new AnvilSaveHandler(new File(getFolderName()),worldName,true);
  }
  int dim;
  WorldInfo worldInfo=savehandler.loadWorldInfo();
  if (worldInfo != null) {
    if (!((WorldProperties)worldInfo).isEnabled()) {
      SpongeMod.instance.getLogger().error("String_Node_Str" + worldName + "String_Node_Str");
      return Optional.absent();
    }
    if (!SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId()).isPresent()) {
      SpongeMod.instance.getSpongeRegistry().registerWorldProperties((WorldProperties)worldInfo);
    }
 else {
      worldInfo=(WorldInfo)SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId()).get();
    }
    dim=((IMixinWorldInfo)worldInfo).getDimensionId();
    if (!DimensionManager.isDimensionRegistered(dim)) {
      DimensionManager.registerDimension(dim,((SpongeDimensionType)((WorldProperties)worldInfo).getDimensionType()).getDimensionTypeId());
    }
    if (SpongeMod.instance.getSpongeRegistry().getWorldFolder(dim) == null) {
      SpongeMod.instance.getSpongeRegistry().registerWorldDimensionId(dim,worldName);
    }
  }
 else {
    return Optional.absent();
  }
  WorldSettings settings=new WorldSettings(worldInfo);
  WorldServer world=(WorldServer)new WorldServer((MinecraftServer)(Object)this,savehandler,worldInfo,dim,this.theProfiler).init();
  world.initialize(settings);
  world.provider.setDimension(dim);
  world.addWorldAccess(new WorldManager((MinecraftServer)(Object)this,world));
  MinecraftForge.EVENT_BUS.post(new WorldEvent.Load(world));
  if (!isSinglePlayer()) {
    world.getWorldInfo().setGameType(getGameType());
  }
  this.setDifficultyForAllWorlds(this.getDifficulty());
  if (((WorldProperties)worldInfo).doesKeepSpawnLoaded()) {
    this.prepareSpawnArea(world);
  }
  return Optional.of((World)world);
}