From TinWiki.org
A technical feature of the MediaWiki software is called "Edit conflict". This will be seen if someone changed the page you were editing, while you were working on it. Without the special feature, you would delete the other's work when you saved your edit. The Edit conflict feature prevents this.
What the feature does, is it notifies you that someone has changed the page since you started writing your edit, and it will show your version in a special section towards the bottom of the page. The new version will be shown in the normal editing window, which means that if you now click the Save button, in the way you would normally do, nothing will happen and the edit you just wrote (the one shown in the lower section) will not be included. To actually get your editing included, in this situation, you have to re-edit the new version. At least, since the feature shows the version text of your edit in the second editing window (the one toward the bottom of the page), you still have that text available.
