Every so often when you save an edit, you get a message saying "Sorry! We could not process your edit due to a loss of session data. Please try again. If it still does not work, try logging out and logging back in." Generally trying again will fix it. It's still bad that this happens because it's very easy to think either that a) your edit was lost irrevocably (it's often not; just resubmit), and b) that you're all done and can close the page.

Recently this has been happening a LOT more than before; I'd guess that the chance I see it on any given edit is 50% or more. 

Warning: completely uninformed speculation follows:
It's almost like the session data is only stored on one of the front-end servers and you have to keep on retrying until you hit the one that knows about you.