en.wp user Ruud Koot reports:

"Occasionally, after a medium-sized editing session, the visual editor will silently refuse to save my edits. When clicking the save button the progress bar appears, disappears, and you stay in editing mode, with no new revision appended to the history. Trying again sometimes resolves the problem, sometimes it requires edit operations to be done in a different order or be broken up into smaller increments, saving in-between."

He also asks, "Do you keep any statistics on how often an editor presses the save button versus how many times this actually results in a new revision being saved?"

Bug 51423 might be related.

Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=51243