Wrong sequence of blocks when using FetchNextBlock

[Roper]

I'm experiencing strange results when using the blockdb and FetchNextBlock to iterate over blocks in my local blocks directory.
My goal is to use the parser of gocoin for a related project and I noticed that the sequence in which blocks are returned by FetchNextBlock is not the necessarily the order given by the blockchain.
The deviation starts at around block #3857 and can be reproduced with this minimal example:

package main

import (
	"log"
	"os"

	"github.com/piotrnar/gocoin/lib/btc"
	"github.com/piotrnar/gocoin/lib/others/blockdb"
)

func main() {
	magic := [4]byte{0xF9, 0xBE, 0xB4, 0xD9}
	db := blockdb.NewBlockDB(os.Args[1], magic)

	for i := 0; i < 4000; i++ {
		dat, _ := db.FetchNextBlock()
		block, _ := btc.NewBlock(dat[:])
		log.Printf("Block #%d hash: %v\n", i, block.Hash)
	}
}

I'm aware that there might be orphaned blocks, however the last two block hashes output by this code are:

Block #3998 hash: 00000000e86a5b1406aec394d058496f0ca3c09ef999e39c2adbd247f3be401f
Block #3999 hash: 000000005b1c6306ad9bf38cdb60642bc24793db171daec4986fc41eb414a7c2

These blocks are #4017 and #4010 in the main chain which means the later block #4017 is processed before #4010!
Any ideas what's going one here? I doubt that this is an actual bug and guess I might just be using it incorrectly. However, I cannot figure out how.
Thanks in advance.

[Piotr Narewski]

It's just returning the blocks in the same order as they are stored on disk.
Sorry, there is nothing I can do here.