Over the last several days, I've been getting sporadic instances (at least five) where I've successfully loaded the read version of a Wikipedia page, but clicking the "Edit" button (or adding the parameter veaction=edit to the URL) just produces the same effect as reloading the read version of a page, but with the proper edit URL.
When I inspect the page, there are a few indications that something different from the read view has been loaded (for example, there's an invisible <div class="ve-init-mw-viewPageTarget-progress">), but not many (for example, ve.dm remains undefined).
Attached is a video of one of these session (warning: it's 47 MB!) on the production English Wikipedia.
Browser: Firefox 39.0a2