Seen as both anon and while logged in:
Edit any page at http://en.wikipedia.beta.wmflabs.org
Use insert menu to insert almost anything: I had the same experience with Formula, Table, and Media.
Save the page after inserting the element.
After save, click the wikitext edit link, whether logged in or anonymous. Nothing happens.
"Invalid _currentGuiderID. Returning early" load.php:153