I just discovered this problem with my MW 1.4.0 site:
* UserA begins editing section1 of a wiki page.
* UserA also begins editing section2 of a wiki page.
* While the section2 edit is still open, section1 edits are saved.
* Section2 edits are then saved.
* The changes made to section1 are lost.
Is this fixed in successive (to 1.4.0) versions of MediaWiki?
-Matt