Program: 27521
Project: 27521
Buggy source:
public static ChangeBlockEvent.Place callBlockPlaceEvent(Event event){
  ChangeBlockEvent.Place spongeEvent=(ChangeBlockEvent.Place)event;
  if (spongeEvent.getCause().root() instanceof Player) {
    EntityPlayer player=(EntityPlayer)spongeEvent.getCause().first(Player.class).get();
    net.minecraft.world.World world=player.world;
    final PhaseTracker phaseTracker=PhaseTracker.getInstance();
    final PhaseContext<?> currentContext=phaseTracker.getCurrentContext();
    PhaseContext<?> target=currentContext;
    if (currentContext instanceof UnwindingPhaseContext) {
      target=((UnwindingPhaseContext)currentContext).getUnwindingContext();
    }
    PacketContext<?> context=target instanceof PacketContext<?> ? (PacketContext<?>)target : null;
    Packet<?> contextPacket=context != null ? context.getPacket() : null;
    if (contextPacket == null) {
      return spongeEvent;
    }
    if (spongeEvent.getTransactions().size() == 1) {
      BlockPos pos=VecHelper.toBlockPos(spongeEvent.getTransactions().get(0).getOriginal().getPosition());
      IBlockState state=(IBlockState)spongeEvent.getTransactions().get(0).getOriginal().getState();
      net.minecraftforge.common.util.BlockSnapshot blockSnapshot=new net.minecraftforge.common.util.BlockSnapshot(world,pos,state);
      IBlockState placedAgainst=Blocks.AIR.getDefaultState();
      EnumHand hand=EnumHand.MAIN_HAND;
      if (contextPacket instanceof CPacketPlayerTryUseItemOnBlock) {
        CPacketPlayerTryUseItemOnBlock packet=(CPacketPlayerTryUseItemOnBlock)contextPacket;
        EnumFacing facing=packet.getDirection();
        placedAgainst=blockSnapshot.getWorld().getBlockState(blockSnapshot.getPos().offset(facing.getOpposite()));
        hand=packet.getHand();
      }
      BlockEvent.PlaceEvent forgeEvent=new BlockEvent.PlaceEvent(blockSnapshot,placedAgainst,player,hand);
      ((IMixinEventBus)MinecraftForge.EVENT_BUS).post(forgeEvent,true);
      if (forgeEvent.isCanceled()) {
        spongeEvent.setCancelled(true);
      }
    }
 else {
      Iterator<Transaction<BlockSnapshot>> iterator=spongeEvent.getTransactions().iterator();
      List<net.minecraftforge.common.util.BlockSnapshot> blockSnapshots=new ArrayList<>();
      while (iterator.hasNext()) {
        Transaction<BlockSnapshot> transaction=iterator.next();
        Location<World> location=transaction.getOriginal().getLocation().get();
        IBlockState state=(IBlockState)transaction.getOriginal().getState();
        BlockPos pos=new BlockPos(location.getBlockX(),location.getBlockY(),location.getBlockZ());
        net.minecraftforge.common.util.BlockSnapshot blockSnapshot=new net.minecraftforge.common.util.BlockSnapshot(world,pos,state);
        blockSnapshots.add(blockSnapshot);
      }
      IBlockState placedAgainst=Blocks.AIR.getDefaultState();
      EnumHand hand=EnumHand.MAIN_HAND;
      if (contextPacket instanceof CPacketPlayerTryUseItemOnBlock) {
        CPacketPlayerTryUseItemOnBlock packet=(CPacketPlayerTryUseItemOnBlock)contextPacket;
        EnumFacing facing=packet.getDirection();
        placedAgainst=blockSnapshots.get(0).getWorld().getBlockState(blockSnapshots.get(0).getPos().offset(facing.getOpposite()));
        hand=packet.getHand();
      }
      BlockEvent.MultiPlaceEvent forgeEvent=new BlockEvent.MultiPlaceEvent(blockSnapshots,placedAgainst,player,hand);
      ((IMixinEventBus)MinecraftForge.EVENT_BUS).post(forgeEvent,true);
      if (forgeEvent.isCanceled()) {
        spongeEvent.setCancelled(true);
      }
    }
  }
  return spongeEvent;
}

Fixed version (ground truth):
private static ChangeBlockEvent.Place callBlockPlaceEvent(Event event){
  ChangeBlockEvent.Place spongeEvent=(ChangeBlockEvent.Place)event;
  if (spongeEvent.getCause().root() instanceof Player) {
    EntityPlayer player=(EntityPlayer)spongeEvent.getCause().root();
    net.minecraft.world.World world=player.world;
    final PhaseTracker phaseTracker=PhaseTracker.getInstance();
    final PhaseContext<?> currentContext=phaseTracker.getCurrentContext();
    PhaseContext<?> target=currentContext;
    if (currentContext instanceof UnwindingPhaseContext) {
      target=((UnwindingPhaseContext)currentContext).getUnwindingContext();
    }
    PacketContext<?> context=target instanceof PacketContext<?> ? (PacketContext<?>)target : null;
    Packet<?> contextPacket=context != null ? context.getPacket() : null;
    if (contextPacket == null) {
      return spongeEvent;
    }
    if (spongeEvent.getTransactions().size() == 1) {
      BlockPos pos=VecHelper.toBlockPos(spongeEvent.getTransactions().get(0).getOriginal().getPosition());
      IBlockState state=(IBlockState)spongeEvent.getTransactions().get(0).getOriginal().getState();
      net.minecraftforge.common.util.BlockSnapshot blockSnapshot=new net.minecraftforge.common.util.BlockSnapshot(world,pos,state);
      IBlockState placedAgainst=Blocks.AIR.getDefaultState();
      EnumHand hand=EnumHand.MAIN_HAND;
      if (contextPacket instanceof CPacketPlayerTryUseItemOnBlock) {
        CPacketPlayerTryUseItemOnBlock packet=(CPacketPlayerTryUseItemOnBlock)contextPacket;
        EnumFacing facing=packet.getDirection();
        placedAgainst=blockSnapshot.getWorld().getBlockState(blockSnapshot.getPos().offset(facing.getOpposite()));
        hand=packet.getHand();
      }
      BlockEvent.PlaceEvent forgeEvent=new BlockEvent.PlaceEvent(blockSnapshot,placedAgainst,player,hand);
      ((IMixinEventBus)MinecraftForge.EVENT_BUS).post(forgeEvent,true);
      if (forgeEvent.isCanceled()) {
        spongeEvent.setCancelled(true);
      }
    }
 else {
      Iterator<Transaction<BlockSnapshot>> iterator=spongeEvent.getTransactions().iterator();
      List<net.minecraftforge.common.util.BlockSnapshot> blockSnapshots=new ArrayList<>();
      while (iterator.hasNext()) {
        Transaction<BlockSnapshot> transaction=iterator.next();
        Location<World> location=transaction.getOriginal().getLocation().get();
        IBlockState state=(IBlockState)transaction.getOriginal().getState();
        BlockPos pos=new BlockPos(location.getBlockX(),location.getBlockY(),location.getBlockZ());
        net.minecraftforge.common.util.BlockSnapshot blockSnapshot=new net.minecraftforge.common.util.BlockSnapshot(world,pos,state);
        blockSnapshots.add(blockSnapshot);
      }
      IBlockState placedAgainst=Blocks.AIR.getDefaultState();
      EnumHand hand=EnumHand.MAIN_HAND;
      if (contextPacket instanceof CPacketPlayerTryUseItemOnBlock) {
        CPacketPlayerTryUseItemOnBlock packet=(CPacketPlayerTryUseItemOnBlock)contextPacket;
        EnumFacing facing=packet.getDirection();
        placedAgainst=blockSnapshots.get(0).getWorld().getBlockState(blockSnapshots.get(0).getPos().offset(facing.getOpposite()));
        hand=packet.getHand();
      }
      BlockEvent.MultiPlaceEvent forgeEvent=new BlockEvent.MultiPlaceEvent(blockSnapshots,placedAgainst,player,hand);
      ((IMixinEventBus)MinecraftForge.EVENT_BUS).post(forgeEvent,true);
      if (forgeEvent.isCanceled()) {
        spongeEvent.setCancelled(true);
      }
    }
  }
  return spongeEvent;
}