This problem arises very often for pages more than 300KB. Every time when you try to edit and save a long page, MediaWiki returns an error. Sometimes even after an error, a new revision appears in the history of the page after some time; sometimes it is not possible to save a page at all.
Today, I had a problem with the following page:
http://uk.wikipedia.org/wiki/Список_відомих_похованих_на_Лук%27янівському_цвинтарі
Version: 1.22.0
Severity: normal
URL: http://uk.wikipedia.org/wiki/Список_відомих_похованих_на_Лук%27янівському_цвинтарі