Randomly?

I don't think this is radomly. If I wait some amount of time, always NWE shows. So probably this is slowly-loading code.

What browser/OS are you using? I haven't counted carefully, but I feel like I'm seeing this more often in Firefox 50 than in Safari 10.

I just opened ten pages via Special:Random at enwiki in each browser. In Safari, all 10 opened in the 2017 wikitext mode. In Firefox, the first opened with no toolbar at all (the 2003 wikitext editor?), the second and eighth in the 2010 WikiEditor, and the other seven opened as expected in the 2017 wikitext mode. One possibility: Does it load the older versions if the page hasn't finished loading (in Read mode) before I clicked the 'Edit source' button?

Using Chrome 55.0.2883.87 m, I never get to use the 2017 wikitext editor when I click on the "Edit wikitext" tab. I always have to click the "Edit wikitext" tab, wait for the 2010 wikitext editor to load, then click again on the "Edit wikitext" tab and I can finally use the 2017 WikiEditor. I've made a few try using Firefox 49.0.2 and I've always got the 2017 WikiEditor.

I have problems only sometime with Chrome. Strange...

I forgot to mention that I have this problem only in ns0, apparently.

I'm using Firefox 50. It does seem to be related to slow-loading code. If I wait to make sure the page has completely loaded, I always get the new 2017 editor.

Yeah, I've tried waiting using Chrome (like 10 minutes) but the new 2017 WikiEditor never loaded, I've always got the 2010 editor.

I've noticed something else: (using Chrome) if I click "Edit source" tab when I'm reading a page I get the 2010 WikiEditor, but if I click "Edito source" from any other tab (History or checking a Diff) I always get the 2017 WikiEditor.

I saw this happening when clicking red links i.e. if I select 'Edit Source' on an existing page I see the NWE, but if I click on a red link it shows me the old wikitext editor.

I can claim this bug I use almost every time keyboard shortcut (alt-shift-e) and it behaves that way really randomly. I think it could be something with namespaces or some different page property, which is more widely expanded through wiki pages

@Dvorapa: Is your account configured to show one tab or two ("Edit" and "Edit source")?

Cswiki users can't have one tab yet. Dvorapa is cswiki user.

In T153561#2916093, @Arrbee wrote:

I saw this happening when clicking red links i.e. if I select 'Edit Source' on an existing page I see the NWE, but if I click on a red link it shows me the old wikitext editor.

Always?

I found out, that if I press alt-shift-e (to edit a page) when being on its ''What links here'' or ''Related chanes'' page, every time it opens OWE instead of NWE (tested today on cswiki, single edit tab is not accessible for this wiki yet)

I have managed to reproduce this after Pearli reported that he gets the issue when editing a talk page, and then clicking the Edit source link for that section again.

It's usually after saving the page and didn't reload it, then clicking edit source again. I think there is a problem with some javascript code.

EDIT:
I've tested other scripts like in gadgets-tab of settings on german wikipedia, and after saving a talk page, none of the scripts start working. Well, i hope it'll help.

I think this bug is common with VisualEditor.

I'm getting this a lot today, and I'm not editing a section. (I have just enabled the 2017WE at ru.wiki, and still get the old editor occasionally.)

I put a few requests for help out.

When I click some link in Wikipedia and then click "edit source" before than the page is fully loaded (under 1 second), it will be opened in the old editor for me. I'm using Firefox. So it's as Kaldari said above. And red links are always opened in the old editor.

This was probably fixed by https://gerrit.wikimedia.org/r/#/c/350582/ as going to ?action=edit will now evaluate on the server side that you actually want the new wikitext editor. (not yet deployed)