The Edit tab for VisualEditor is sometimes not correctly initialized. Loading about 50 random pages, on one of them I will not have the "Edit source" tab, and the "Edit" tab will point to the wikitext editor instead of VisualEditor.
Chrome/Ubuntu, logged-in. Console shows the following error when this occurs:
Exception thrown by skins.vector.js: Object [object Object] has no method 'collapsibleTabs'
Version: unspecified
Severity: normal