Program: 169649
Project: 169649
Buggy source:
/** 
 * Assumes canAddItem was already called, but still checks for sanity at some level. Will always consume item as it only checks type.
 * @param stack - item being added
 * @return true if the item was accepted
 */
public boolean addItem(final ItemStack stack){
  if (stack != null) {
    if (isRod(stack) && !frameCompleted) {
      int added=addRods(stack.stackSize);
      stack.stackSize-=added;
      return added > 0;
    }
 else     if (isPlate(stack) && !skinCompleted) {
      int added=addPlates(stack.stackSize);
      stack.stackSize-=added;
      return added > 0;
    }
 else {
      Item item=stack.getItem();
      boolean added=false;
      if (item instanceof IRocketEngine) {
        if (((IMissileModule)item).getMissileSize() == MissileCasings.STANDARD.ordinal()) {
          if (rocketEngine == null) {
            rocketEngine=stack.copy();
            rocketEngine.stackSize=1;
            stack.stackSize--;
            added=true;
          }
        }
      }
 else       if (item instanceof IWarhead) {
        if (warhead == null) {
          warhead=stack.copy();
          warhead.stackSize=1;
          stack.stackSize--;
          added=true;
        }
      }
 else       if (item instanceof IGuidance) {
        if (rocketComputer == null) {
          rocketComputer=stack.copy();
          rocketComputer.stackSize=1;
          stack.stackSize--;
          added=true;
        }
      }
      if (rocketComputer != null && rocketEngine != null) {
        this.gutsCompleted=true;
      }
      return added;
    }
  }
  return false;
}

Fixed version (ground truth):
/** 
 * Assumes canAddItem was already called, but still checks for sanity at some level. Will always consume item as it only checks type.
 * @param stack - item being added
 * @return true if the item was accepted
 */
public boolean addItem(final ItemStack stack){
  if (stack != null) {
    if (isRod(stack) && !frameCompleted) {
      int added=addRods(stack.stackSize);
      stack.stackSize-=added;
      return added > 0;
    }
 else     if (isPlate(stack) && !skinCompleted) {
      int added=addPlates(stack.stackSize);
      stack.stackSize-=added;
      return added > 0;
    }
 else {
      Item item=stack.getItem();
      boolean added=false;
      if (item instanceof IModuleItem) {
        IModule module=((IModuleItem)item).getModule(stack);
        if (module instanceof IRocketEngine) {
          if (((IMissileModule)item).getMissileSize() == MissileCasings.STANDARD.ordinal()) {
            if (rocketEngine == null) {
              rocketEngine=stack.copy();
              rocketEngine.stackSize=1;
              stack.stackSize--;
              added=true;
            }
          }
        }
 else         if (module instanceof IWarhead) {
          if (warhead == null) {
            warhead=stack.copy();
            warhead.stackSize=1;
            stack.stackSize--;
            added=true;
          }
        }
 else         if (module instanceof IGuidance) {
          if (rocketComputer == null) {
            rocketComputer=stack.copy();
            rocketComputer.stackSize=1;
            stack.stackSize--;
            added=true;
          }
        }
        if (rocketComputer != null && rocketEngine != null) {
          this.gutsCompleted=true;
        }
        return added;
      }
    }
  }
  return false;
}