When loading the visual editor, the session is not (re)started and it continues the one from when you opened the editor in read mode. When the session expires the edits cannot be saved ("Error: We could not process your edit because your session has expired.")
Normally this isn't a problem, except it's counter intuitive (and apparently counter to the way the source editor works) when a page has been open for an extended time prior to entering the edit mode as it apparently continues that session rather than starting a new one. Unlike in source editor (which does work after an extended period in read mode) there is no possibility of working around a session expiry by clicking save again - the page must be reloaded and the edit made again by hand.
Steps to reproduce:
- Open any VE-editable page in read mode
- Wait until the session expires*
- Launch the visual editor
- Make a change and attempt to save the page
*I don't know how long this is but I encountered the error on a page open for 625 minutes (10.4 hours).
Bug 50424 is possibly related, but the person who reported this issue on en.wikipedia doesn't think so.