Program: 11755
Project: 11755
Buggy source:
@Override public void handleReceived(ClientConnection connection){
  TridentPlayer player=((PlayerConnection)connection).player();
  location.setWorld(player.world());
  Block clickAt=location.block();
  if (location.y() < 255 && location.block() != null && clickAt.substance().isFunctional() && !player.isCrouching()) {
switch (clickAt.substance()) {
case FURNACE:
case BURNING_FURNACE:
      ((FurnaceMetaImpl)clickAt.obtainMeta(FurnaceMeta.class)).furnaceInventory().sendTo(player);
    break;
case CHEST:
  ((TridentInventory)clickAt.obtainMeta(ChestMeta.class).inventory()).sendTo(player);
break;
}
}
 else if (player.heldItem() != null && player.heldItem().type() != Substance.AIR) {
Substance substance=player.heldItem().type();
Vector vector=determineOffset();
if (!substance.isBlock()) {
return;
}
if (location.y() + vector.y() > 255 || location.y() + vector.y() < 0) {
return;
}
Position position=clickAt.substance().canBeReplaced() ? location : location.relative(vector);
Block block=new OwnedTridentBlock(player,position.block());
short yaw=(short)(player.position().yaw() * 10);
short meta=player.heldItem().damageValue();
Value<Byte> result=Value.of((byte)0);
Value<Substance> substanceValue=Value.of(substance);
boolean allow=MetaFactory.decode(block,substanceValue,new byte[]{writeFirst(yaw),writeSecond(yaw),direction,((byte)cursorPosition.x()),((byte)cursorPosition.y()),((byte)cursorPosition.z()),writeFirst(meta),writeSecond(meta)},result);
if (allow) {
block.setSubstanceAndMeta(substanceValue.get(),result.get());
SoundEffectType soundEffectType=substanceValue.get().placeSound();
if (soundEffectType != null) {
SoundEffect sound=location.world().playSound(soundEffectType);
sound.setPosition(location);
sound.apply();
}
}
}
}

Fixed version (ground truth):
@Override public void handleReceived(ClientConnection connection){
  TridentPlayer player=((PlayerConnection)connection).player();
  location.setWorld(player.world());
  Substance substance=player.heldItem().type();
  Vector vector=determineOffset();
  if (!substance.isBlock()) {
    return;
  }
  if (location.y() + vector.y() > 255 || location.y() + vector.y() < 0) {
    return;
  }
  Position position=location.block().substance().canBeReplaced() ? location : location.relative(vector);
  Block block=new OwnedTridentBlock(player,position.block());
  if (location.y() < 255 && location.block() != null && block.substance().isFunctional() && !player.isCrouching()) {
switch (block.substance()) {
case FURNACE:
case BURNING_FURNACE:
      ((FurnaceMetaImpl)block.obtainMeta(FurnaceMeta.class)).furnaceInventory().sendTo(player);
    break;
case CHEST:
  ((TridentInventory)block.obtainMeta(ChestMeta.class).inventory()).sendTo(player);
break;
}
}
 else if (player.heldItem() != null && player.heldItem().type() != Substance.AIR) {
System.out.println(position);
short yaw=(short)(player.position().yaw() * 10);
short meta=player.heldItem().damageValue();
Value<Byte> result=Value.of((byte)0);
Value<Substance> substanceValue=Value.of(substance);
boolean allow=MetaFactory.decode(block,substanceValue,new byte[]{writeFirst(yaw),writeSecond(yaw),direction,((byte)cursorPosition.x()),((byte)cursorPosition.y()),((byte)cursorPosition.z()),writeFirst(meta),writeSecond(meta)},result);
if (allow) {
block.setSubstanceAndMeta(substanceValue.get(),result.get());
SoundEffectType soundEffectType=substanceValue.get().placeSound();
if (soundEffectType != null) {
SoundEffect sound=location.world().playSound(soundEffectType);
sound.setPosition(location);
sound.apply();
}
}
}
}