Program: 34766
Project: 34766
Buggy source:
public void updateCore(World world,int x,int y,int z,int meta){
  if (world.playerEntities.isEmpty())   return;
  if (world.rand.nextInt(3) == 0) {
    int halfsz=ComponentIsland.halfSize, playerCheck=halfsz * 2;
    SpawnEntry entry=spawnEntries.containsKey((byte)meta) ? spawnEntries.get((byte)meta).getRandomItem(world.rand) : null;
    if (entry == null)     return;
    if (world.difficultySetting == EnumDifficulty.PEACEFUL && entry.isMob)     return;
    int currentAmount=world.getEntitiesWithinAABB(entry.getMobClass(),AxisAlignedBB.getBoundingBox(x - halfsz,y + 10,z - halfsz,x + halfsz,y + 55,z + halfsz)).size();
    if (currentAmount >= entry.getMaxAmount() || world.rand.nextFloat() * 1.1F < (float)currentAmount / entry.getMaxAmount())     return;
    int playerAmount=world.playerEntities.size();
    for (int a=0; a < Math.min(entry.getMaxAmount() >> 2,world.rand.nextInt(1 + entry.getMaxAmount() - currentAmount)); a++) {
      EntityLiving e=entry.createMob(world);
      for (int attempt=0; attempt < 20 + Math.min(10,playerAmount); attempt++) {
        EntityPlayer player=(EntityPlayer)world.playerEntities.get(world.rand.nextInt(playerAmount));
        if (MathUtil.distance(player.posX - x,player.posZ - z) > playerCheck)         continue;
        double ang=world.rand.nextDouble() * 2D * Math.PI, len=19 + world.rand.nextInt(55) + Math.abs(world.rand.nextGaussian() * 12D);
        double posX=player.posX + Math.cos(ang) * len, posZ=player.posZ + Math.sin(ang) * len;
        for (int yAttempt=0; yAttempt < 28; yAttempt++) {
          e.setLocationAndAngles(posX,(int)Math.floor(player.posY + (world.rand.nextDouble() - 0.65D) * (yAttempt + 4) * 3D) + 0.01D,posZ,world.rand.nextFloat() * 360F,0F);
          if (hasEntitySpace(world,e))           break;
        }
        int xx=(int)Math.floor(e.posX), zz=(int)Math.floor(e.posZ);
        boolean hasBlockBelow=false;
        for (int yy=(int)e.posY - 1; yy > e.posY - 4D; yy--) {
          if (world.getBlock(xx,yy,zz).isOpaqueCube()) {
            hasBlockBelow=true;
            break;
          }
        }
        if (!hasBlockBelow || DragonUtil.getTopBlock(world,BlockList.end_terrain,xx,zz,120) <= 30)         continue;
        e.setPositionAndUpdate(e.posX,e.posY + 0.01D,e.posZ);
        world.spawnEntityInWorld(e);
        break;
      }
    }
  }
  if (world.rand.nextFloat() < getInteractionChance(contentVariations.get(meta)) && interactions.containsKey((byte)meta) && world.getEntitiesWithinAABB(EntityTechnicalBiomeInteraction.class,AxisAlignedBB.getBoundingBox(x - 1,y - 1,z - 1,x + 2,y + 2,z + 2)).isEmpty()) {
    AbstractBiomeInteraction interaction=interactions.get((byte)meta).getRandomItem(world.rand).create();
    if (interaction != null)     world.spawnEntityInWorld(new EntityTechnicalBiomeInteraction(world,x + 0.5D,y + 0.5D,z + 0.5D,interaction));
  }
  for (  Object o : world.playerEntities) {
    EntityPlayer player=(EntityPlayer)o;
    if (isPlayerMoving(player)) {
      int ix=(int)player.posX, iy=(int)player.posY - 1, iz=(int)player.posZ;
      if (world.getBlock(ix,iy,iz) == getTopBlock() && world.getBlockMetadata(ix,iy,iz) == getTopBlockMeta()) {
        player.addStat(AchievementManager.WHOLE_NEW_CULTURES,1);
      }
    }
  }
}

Fixed version (ground truth):
public void updateCore(World world,int x,int y,int z,int meta){
  if (world.playerEntities.isEmpty())   return;
  if (world.rand.nextInt(3) == 0) {
    int halfsz=ComponentIsland.halfSize, playerCheck=halfsz * 2;
    SpawnEntry entry=spawnEntries.containsKey((byte)meta) ? spawnEntries.get((byte)meta).getRandomItem(world.rand) : null;
    if (entry == null)     return;
    if (world.difficultySetting == EnumDifficulty.PEACEFUL && entry.isMob)     return;
    int currentAmount=world.getEntitiesWithinAABB(entry.getMobClass(),AxisAlignedBB.getBoundingBox(x - halfsz,y + 10,z - halfsz,x + halfsz,y + 55,z + halfsz)).size();
    if (currentAmount >= entry.getMaxAmount() || world.rand.nextFloat() * 1.1F < (float)currentAmount / entry.getMaxAmount())     return;
    int playerAmount=world.playerEntities.size();
    for (int a=0; a < Math.min(entry.getMaxAmount() >> 2,world.rand.nextInt(1 + entry.getMaxAmount() - currentAmount)); a++) {
      EntityLiving e=entry.createMob(world);
      for (int attempt=0; attempt < 20 + Math.min(10,playerAmount); attempt++) {
        EntityPlayer player=(EntityPlayer)world.playerEntities.get(world.rand.nextInt(playerAmount));
        if (MathUtil.distance(player.posX - x,player.posZ - z) > playerCheck)         continue;
        double ang=world.rand.nextDouble() * 2D * Math.PI, len=19 + world.rand.nextInt(55) + Math.abs(world.rand.nextGaussian() * 12D);
        double posX=player.posX + Math.cos(ang) * len, posZ=player.posZ + Math.sin(ang) * len;
        for (int yAttempt=0; yAttempt < 28; yAttempt++) {
          e.setLocationAndAngles(posX,(int)Math.floor(player.posY + (world.rand.nextDouble() - 0.65D) * (yAttempt + 4) * 3D) + 0.01D,posZ,world.rand.nextFloat() * 360F,0F);
          if (hasEntitySpace(world,e))           break;
        }
        int xx=(int)Math.floor(e.posX), zz=(int)Math.floor(e.posZ);
        boolean hasBlockBelow=false;
        for (int yy=(int)e.posY - 1; yy > e.posY - 4D; yy--) {
          if (world.getBlock(xx,yy,zz).isOpaqueCube()) {
            hasBlockBelow=true;
            break;
          }
        }
        if (!hasBlockBelow || DragonUtil.getTopBlock(world,BlockList.end_terrain,xx,zz,120) <= 30)         continue;
        e.setPositionAndUpdate(e.posX,e.posY + 0.01D,e.posZ);
        world.spawnEntityInWorld(e);
        break;
      }
    }
  }
  BiomeContentVariation variation=null;
  for (  BiomeContentVariation contentVariation : contentVariations) {
    if (contentVariation.id == meta) {
      variation=contentVariation;
      break;
    }
  }
  if (variation != null && world.rand.nextFloat() < getInteractionChance(variation) && interactions.containsKey(variation.id) && world.getEntitiesWithinAABB(EntityTechnicalBiomeInteraction.class,AxisAlignedBB.getBoundingBox(x - 1,y - 1,z - 1,x + 2,y + 2,z + 2)).isEmpty()) {
    AbstractBiomeInteraction interaction=interactions.get(variation.id).getRandomItem(world.rand).create();
    if (interaction != null)     world.spawnEntityInWorld(new EntityTechnicalBiomeInteraction(world,x + 0.5D,y + 0.5D,z + 0.5D,interaction));
  }
  for (  Object o : world.playerEntities) {
    EntityPlayer player=(EntityPlayer)o;
    if (isPlayerMoving(player)) {
      int ix=(int)player.posX, iy=(int)player.posY - 1, iz=(int)player.posZ;
      if (world.getBlock(ix,iy,iz) == getTopBlock() && world.getBlockMetadata(ix,iy,iz) == getTopBlockMeta()) {
        player.addStat(AchievementManager.WHOLE_NEW_CULTURES,1);
      }
    }
  }
}