Thanks so much guys! 
That was that problem.

I replaced [./img] blocks by [./code] blocks.

Problem solved!

I'm writing a very long post with many images and [.code] blocks.
But when I finished it, I got a [.code][.code] block that spawned at the top from out of nowhere.

That is the not the first time I created a thread, I know how these blocks works. I re-read my thread many times and I don't see where is the problem.
I tried to write random letters at the very beginning of my thread and after that, I saw [.code]randomword[.code].

This is very weird and the first time I saw that. How can I fix it ?

Thanks.

NB: I perfectly know that [.code] is written without a dot.