svwiki users NH and Larske report at https://sv.wikipedia.org/wiki/Wikipedia:Wikipediafr%C3%A5gor#Fliken_.22Redigera.22_.28dvs_Visual_Editor.29_finns_ibland_inte that VisualEditor is sometimes not available.
The message in the console is "Your skin is incompatible with VisualEditor. See https://www.mediawiki.org/wiki/VisualEditor/Skin_requirements for the requirements.".
This only happens sometimes (one in 5, or one in 10 page loads), indicating a race condition.
It can be added the same thing happens when logged out (in a new browser session), so it should not have to do with user specific settings etc.
When logged in, Visual Editor seems absent in about 1 of 5 times. When not logged in the error appears less frequently, maybe 1 in 20 or 50 times.
For me this happens in Safari 10.1. User:Larske reports other web browsers as well.
I cannot replicate this on en.wikipedia.org or fr.wikipedia.org.
I saw this happening very reliably on it.wikipedia.org when looking at another bug report.
It's possible that a user script or gadget is re-writing the page and so sometimes (depending on timing) when VisualEditor looks at the page it looks like we won't be able to load?
Change 350471 had a related patch set uploaded (by Bartosz Dziewoński):
[mediawiki/extensions/VisualEditor@master] init: Relax checks for required skin elements to avoid false positives
As @PerfektesChaos noticed on T164061, it looks like this is a race condition caused by not waiting for DOM ready.
Change 350471 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] init: Fix checks for required skin elements to avoid false positives
Change 351282 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/extensions/VisualEditor@REL1_29] init: Fix checks for required skin elements to avoid false positives
Change 351282 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@REL1_29] init: Fix checks for required skin elements to avoid false positives