I am running Emacs 25 on Ubuntu 18.04. When I type accents, they display properly, but when I close and then reopen the file they appear like a backslash and then an octal number.
This only happens with one particular file: all other files I process with Emacs on the same computer and same user account save and display accents fine. How is that possible? Whatever settings are contained in the `.emacs` config file would apply to all files?
I've tried to set the coding system on the individual file that is causing problems, with the command `set-buffer-file-coding-system` and then selecting `utf-8-unix`, but I get the message: `utf-8-unix cannot encode these: é`

How can I fix the encoding of this file?




@DoMiNeLa10 Thanks for your reply.
Nothing particular with file name, nor comments and the major mode is org-mode.
What I suspect that has happened is that the problematic file has weird characters from copy/pastes from webpages.
For instance, it contains the octal sequence `\342\200\224` which on the original web page looks like a long dash. I suspect emacs doesn't know how to display it and then gets confused and gives up altogether trying to figure out the encoding; For instance, the file is full of the octal sequence `\303\251` which is just accented é, that emacs should know how to display (as I said in my previous post I tried the command `set-buffer-file-coding-system` and then selected `utf-8-unix`, but it continues to display accents as octal sequences)

Is there a way to force emacs to use the utf-8-unix encoding to display accents correctly and just ignore the octal sequences that are not part of the encoding? I mean, I don't mind that the weird characters just remain octal sequences in the file, but I want to see the accents.

@Stefan: the output of checking: `M-L buffer-file-coding-system RET`

 buffer-file-coding-system is a variable defined in ‘C source code’.
 Its value is ‘no-conversion’
 Local in buffer todo1.org; global value is utf-8-unix
 Automatically becomes buffer-local when set.

...

I tried `C-x RET r utf-8 RET` as you suggested and it works well, displaying all accents correctly instead of backslash octal sequences and the `buffer-file-coding-system` value becomes `utf-8`, but if I close the file and then open again, the backslash octal mess reappears and the value reverts to '`no-conversion`'.

@DoMiNeLa10 I `ctrl-c` to copy from firefox and `ctrl-y` to paste in emacs. Sometimes, I just select in firefox and then middle-click mouse button to paste. Maybe pasting with the middle-click mouse button is the cause of the problem?