Program: 156534
Project: 156534
Buggy source:
/** 
 * Gets the next available block. If builder is not null, then building will be verified and performed. Otherwise, the next possible building slot is returned, possibly for reservation, with no building.
 */
private BuildingSlot internalGetNextBlock(World world,TileAbstractBuilder builder){
  if (builder != null && builder.energyAvailable() < BuilderAPI.BREAK_ENERGY) {
    return null;
  }
  iterator=new BuildingSlotMapIterator(buildList,builder,buildStageOccurences);
  BuildingSlotBlock slot;
  while ((slot=iterator.next()) != null) {
    if (!world.blockExists(slot.x,slot.y,slot.z)) {
      continue;
    }
    boolean skipped=false;
    for (int i=0; i < slot.buildStage; i++) {
      if (buildStageOccurences[i] > 0) {
        iterator.skipList();
        skipped=true;
        break;
      }
    }
    if (skipped) {
      continue;
    }
    if (slot.built) {
      iterator.remove();
      markLocationUsed(slot.x,slot.y,slot.z);
      postProcessing.add(slot);
      continue;
    }
    if (slot.reserved) {
      continue;
    }
    try {
      if (slot.isAlreadyBuilt(context)) {
        requirementMap.remove(slot);
        if (slot.mode == Mode.Build) {
          postProcessing.add(slot);
        }
        iterator.remove();
        continue;
      }
      if (BlockUtils.isUnbreakableBlock(world,slot.x,slot.y,slot.z)) {
        iterator.remove();
        markLocationUsed(slot.x,slot.y,slot.z);
        requirementMap.remove(slot);
      }
 else {
        if (slot.mode == Mode.ClearIfInvalid) {
          if (BuildCraftAPI.isSoftBlock(world,slot.x,slot.y,slot.z) || isBlockBreakCanceled(world,slot.x,slot.y,slot.z)) {
            iterator.remove();
            markLocationUsed(slot.x,slot.y,slot.z);
            requirementMap.remove(slot);
          }
 else {
            if (builder == null) {
              createDestroyItems(slot);
              return slot;
            }
 else             if (canDestroy(builder,context,slot)) {
              consumeEnergyToDestroy(builder,slot);
              createDestroyItems(slot);
              iterator.remove();
              markLocationUsed(slot.x,slot.y,slot.z);
              requirementMap.remove(slot);
              return slot;
            }
          }
        }
 else         if (!slot.schematic.doNotBuild()) {
          if (builder == null) {
            return slot;
          }
 else           if (checkRequirements(builder,slot.schematic)) {
            if (!BuildCraftAPI.isSoftBlock(world,slot.x,slot.y,slot.z) || requirementMap.contains(new BlockIndex(slot.x,slot.y,slot.z))) {
              continue;
            }
 else             if (isBlockPlaceCanceled(world,slot.x,slot.y,slot.z,slot.schematic)) {
              iterator.remove();
              requirementMap.remove(slot);
              markLocationUsed(slot.x,slot.y,slot.z);
              continue;
            }
            builder.consumeEnergy(slot.getEnergyRequirement());
            useRequirements(builder,slot);
            iterator.remove();
            markLocationUsed(slot.x,slot.y,slot.z);
            postProcessing.add(slot);
            return slot;
          }
        }
 else {
          postProcessing.add(slot);
          requirementMap.remove(slot);
          iterator.remove();
        }
      }
    }
 catch (    Throwable t) {
      t.printStackTrace();
      BCLog.logger.throwing(t);
      iterator.remove();
      requirementMap.remove(slot);
    }
  }
  return null;
}

Fixed version (ground truth):
/** 
 * Gets the next available block. If builder is not null, then building will be verified and performed. Otherwise, the next possible building slot is returned, possibly for reservation, with no building.
 */
private BuildingSlot internalGetNextBlock(World world,TileAbstractBuilder builder){
  if (builder != null && builder.energyAvailable() < BuilderAPI.BREAK_ENERGY) {
    return null;
  }
  iterator=new BuildingSlotMapIterator(buildList,builder,buildStageOccurences);
  BuildingSlotBlock slot;
  while ((slot=iterator.next()) != null) {
    if (!world.blockExists(slot.x,slot.y,slot.z)) {
      continue;
    }
    boolean skipped=false;
    for (int i=0; i < slot.buildStage; i++) {
      if (buildStageOccurences[i] > 0) {
        iterator.skipList();
        skipped=true;
        break;
      }
    }
    if (skipped) {
      continue;
    }
    if (slot.built) {
      iterator.remove();
      markLocationUsed(slot.x,slot.y,slot.z);
      postProcessing.add(slot);
      continue;
    }
    if (slot.reserved) {
      continue;
    }
    try {
      if (slot.isAlreadyBuilt(context)) {
        if (slot.mode == Mode.Build) {
          requirementMap.remove(slot);
          postProcessing.add(slot);
        }
        iterator.remove();
        continue;
      }
      if (BlockUtils.isUnbreakableBlock(world,slot.x,slot.y,slot.z)) {
        iterator.remove();
        markLocationUsed(slot.x,slot.y,slot.z);
        requirementMap.remove(slot);
      }
 else {
        if (slot.mode == Mode.ClearIfInvalid) {
          if (BuildCraftAPI.isSoftBlock(world,slot.x,slot.y,slot.z) || isBlockBreakCanceled(world,slot.x,slot.y,slot.z)) {
            iterator.remove();
            markLocationUsed(slot.x,slot.y,slot.z);
          }
 else {
            if (builder == null) {
              createDestroyItems(slot);
              return slot;
            }
 else             if (canDestroy(builder,context,slot)) {
              consumeEnergyToDestroy(builder,slot);
              createDestroyItems(slot);
              iterator.remove();
              markLocationUsed(slot.x,slot.y,slot.z);
              return slot;
            }
          }
        }
 else         if (!slot.schematic.doNotBuild()) {
          if (builder == null) {
            return slot;
          }
 else           if (checkRequirements(builder,slot.schematic)) {
            if (!BuildCraftAPI.isSoftBlock(world,slot.x,slot.y,slot.z) || requirementMap.contains(new BlockIndex(slot.x,slot.y,slot.z))) {
              continue;
            }
 else             if (isBlockPlaceCanceled(world,slot.x,slot.y,slot.z,slot.schematic)) {
              iterator.remove();
              requirementMap.remove(slot);
              markLocationUsed(slot.x,slot.y,slot.z);
              continue;
            }
            builder.consumeEnergy(slot.getEnergyRequirement());
            useRequirements(builder,slot);
            iterator.remove();
            markLocationUsed(slot.x,slot.y,slot.z);
            postProcessing.add(slot);
            return slot;
          }
        }
 else {
          postProcessing.add(slot);
          requirementMap.remove(slot);
          iterator.remove();
        }
      }
    }
 catch (    Throwable t) {
      t.printStackTrace();
      BCLog.logger.throwing(t);
      iterator.remove();
      requirementMap.remove(slot);
    }
  }
  return null;
}