Program: 109764
Project: 109764
Buggy source:
@Override public boolean makePortal(Entity entity){
  System.out.println("String_Node_Str");
  int searchArea=16;
  double closestSpot=-1D;
  int entityX=MathHelper.floor(entity.posX);
  int entityY=MathHelper.floor(entity.posY);
  int entityZ=MathHelper.floor(entity.posZ);
  int foundX=entityX;
  int foundY=entityY;
  int foundZ=entityZ;
  for (int x=entityX - searchArea; x <= entityX + searchArea; x++) {
    double distX=(x + 0.5D) - entity.posX;
    nextCoords:     for (int z=entityZ - searchArea; z <= entityZ + searchArea; z++) {
      double distZ=(z + 0.5D) - entity.posZ;
      int y=world.getHeight() - 1;
      BlockPos pos=new BlockPos(x,y,z);
      for (; y >= 63 - 1 && (world.getBlockState(pos).getBlock() == Blocks.AIR || !world.getBlockState(pos).isOpaqueCube()); pos=pos.down()) {
        y=pos.getY();
      }
      if (y > 63 + 20 || y < 63) {
        continue;
      }
      BlockPos tryPos=new BlockPos(x,y,z);
      if (getValidBuildBlocks().contains(world.getBlockState(tryPos))) {
        for (int xOffset=-2; xOffset <= 2; xOffset++) {
          for (int zOffset=-2; zOffset <= 2; zOffset++) {
            int otherY=world.getHeight() - 1;
            BlockPos pos1=new BlockPos(x + xOffset,otherY,z + zOffset);
            BlockPos pos2=tryPos.toImmutable();
            for (; otherY >= 63 && (world.getBlockState(pos1).getBlock() == Blocks.AIR || !world.getBlockState(pos2).isOpaqueCube()); pos1=pos1.down()) {
              otherY=pos1.getY();
            }
            if (Math.abs(y - otherY) >= 3) {
              continue nextCoords;
            }
          }
        }
        double distY=(y + 0.5D) - entity.posY;
        double distance=distX * distX + distY * distY + distZ * distZ;
        if (closestSpot < 0.0D || distance < closestSpot) {
          closestSpot=distance;
          foundX=x;
          foundY=y;
          foundZ=z;
        }
      }
    }
  }
  int worldSpawnX=MathHelper.floor(foundX);
  int worldSpawnZ=MathHelper.floor(foundZ);
  int worldSpawnY=getTerrainHeightAt(worldSpawnX,worldSpawnZ);
  int SEARCH_FOR_LAND_DISTANCE_MAX=200;
  if (closestSpot < 0.0D) {
    foundY=worldSpawnY - 2;
    boolean foundLand=false;
    for (int dist=1; !foundLand && dist < SEARCH_FOR_LAND_DISTANCE_MAX; dist++) {
      for (      EnumFacing dir : EnumFacing.HORIZONTALS) {
        BlockPos pos=new BlockPos(worldSpawnX,worldSpawnY,worldSpawnZ).offset(dir,3 + dist);
        IBlockState state=world.getBlockState(pos);
        if (getValidBuildBlocks().contains(state)) {
          foundLand=true;
          BlockPos buildpos=new BlockPos(worldSpawnX,worldSpawnY + 1,worldSpawnZ).offset(dir,3);
          while (!buildpos.equals(pos.up())) {
            world.setBlockState(buildpos,thatchBlock);
            world.setBlockState(buildpos.offset(dir.rotateY()),thatchBlock);
            world.setBlockState(buildpos.offset(dir.rotateYCCW()),thatchBlock);
            buildpos=buildpos.offset(dir);
          }
          BlockPos stairPosMid=new BlockPos(pos.getX(),worldSpawnY + 1,worldSpawnZ);
          placeStairs(stairPosMid,dir.getOpposite());
          generateThatchBorder(worldSpawnX,worldSpawnY + 1,worldSpawnZ);
          break;
        }
      }
    }
  }
  entity.setLocationAndAngles(foundX,foundY + 2,foundZ,entity.rotationYaw,0.0F);
  buildTeleporterAt(worldSpawnX,worldSpawnY + 1,worldSpawnZ,entity);
  System.out.println("String_Node_Str");
  return true;
}

Fixed version (ground truth):
@Override public boolean makePortal(Entity entity){
  System.out.println("String_Node_Str");
  int searchArea=16;
  double closestSpot=-1D;
  int entityX=MathHelper.floor(entity.posX);
  int entityY=MathHelper.floor(entity.posY);
  int entityZ=MathHelper.floor(entity.posZ);
  int foundX=entityX;
  int foundY=entityY;
  int foundZ=entityZ;
  for (int x=entityX - searchArea; x <= entityX + searchArea; x++) {
    double distX=(x + 0.5D) - entity.posX;
    nextCoords:     for (int z=entityZ - searchArea; z <= entityZ + searchArea; z++) {
      double distZ=(z + 0.5D) - entity.posZ;
      int y=world.getHeight() - 1;
      BlockPos pos=new BlockPos(x,y,z);
      for (; y >= 63 - 1 && (world.getBlockState(pos).getBlock() == Blocks.AIR || !getValidBuildBlocks().contains(world.getBlockState(pos))); pos=pos.down()) {
        y=pos.getY();
      }
      if (y > 63 + 20 || y < 63) {
        continue;
      }
      BlockPos tryPos=new BlockPos(x,y,z);
      if (getValidBuildBlocks().contains(world.getBlockState(tryPos))) {
        for (int xOffset=-2; xOffset <= 2; xOffset++) {
          for (int zOffset=-2; zOffset <= 2; zOffset++) {
            int otherY=world.getHeight() - 1;
            BlockPos pos1=new BlockPos(x + xOffset,otherY,z + zOffset);
            BlockPos pos2=tryPos.toImmutable();
            for (; otherY >= 63 && (world.getBlockState(pos1).getBlock() == Blocks.AIR || !world.getBlockState(pos2).isOpaqueCube()); pos1=pos1.down()) {
              otherY=pos1.getY();
            }
            if (Math.abs(y - otherY) >= 3) {
              continue nextCoords;
            }
          }
        }
        double distY=(y + 0.5D) - entity.posY;
        double distance=distX * distX + distY * distY + distZ * distZ;
        if (closestSpot < 0.0D || distance < closestSpot) {
          closestSpot=distance;
          foundX=x;
          foundY=y;
          foundZ=z;
        }
      }
    }
  }
  int worldSpawnX=MathHelper.floor(foundX);
  int worldSpawnZ=MathHelper.floor(foundZ);
  int worldSpawnY=getTerrainHeightAt(worldSpawnX,worldSpawnZ);
  int SEARCH_FOR_LAND_DISTANCE_MAX=200;
  if (closestSpot < 0.0D) {
    foundY=worldSpawnY - 2;
    boolean foundLand=false;
    for (int dist=1; !foundLand && dist < SEARCH_FOR_LAND_DISTANCE_MAX; dist++) {
      for (      EnumFacing dir : EnumFacing.HORIZONTALS) {
        BlockPos pos=new BlockPos(worldSpawnX,worldSpawnY,worldSpawnZ).offset(dir,3 + dist);
        IBlockState state=world.getBlockState(pos);
        if (getValidBuildBlocks().contains(state)) {
          foundLand=true;
          BlockPos buildpos=new BlockPos(worldSpawnX,worldSpawnY + 1,worldSpawnZ).offset(dir,3);
          while (!buildpos.equals(pos.up())) {
            world.setBlockState(buildpos,thatchBlock);
            world.setBlockState(buildpos.offset(dir.rotateY()),thatchBlock);
            world.setBlockState(buildpos.offset(dir.rotateYCCW()),thatchBlock);
            buildpos=buildpos.offset(dir);
          }
          BlockPos stairPosMid=new BlockPos(pos.getX(),worldSpawnY + 1,worldSpawnZ);
          placeStairs(stairPosMid,dir.getOpposite());
          generateThatchBorder(worldSpawnX,worldSpawnY + 1,worldSpawnZ);
          break;
        }
      }
    }
  }
  entity.setLocationAndAngles(foundX,foundY + 2,foundZ,entity.rotationYaw,0.0F);
  buildTeleporterAt(worldSpawnX,worldSpawnY + 1,worldSpawnZ,entity);
  System.out.println("String_Node_Str");
  return true;
}