VE's help icon popup includes this "visualeditor-beta-warning" text:
... To switch back to source editing at any time without losing your changes, open the dropdown next to "Cancel" and select "Switch to source editing?"."
The last message is 'visualeditor-mweditmodesource-title'. But the item in the dropdown is actually visualeditor-mweditmodesource-tool, which is "Switch to source editing" without a question mark. Trivial to fix.
(Also "Cancel" is gone from the VE UI, I filed bug 69201.)