User blog comment:Dopp/Technical Update: November 29, 2011/@comment-1331296-20111202182242/@comment-13301-20111202183702

That's new in the new editor? Because edit conflict detection on MediaWiki is done generating a "start timestamp" in the edit form that remain constant while you preview and when you save it checks if there was an edit after that "start timestamp". If no edit was made, no edit conflict. If an edit was made, it will try to automatically merge your changes into the last revision. If the automatic merge fails then you get the edit conflict. But it doesn't trigger edit conflict if you hit the edit button twice, unless you attempt to make both edits in parallel (different windows/tabs).

With the RTE editor edit conflicts are more likely to happen since the editor changes random spaces, sometimes depending on the browser being used, and if someone is editing at the same time and both change the same lines then you get the edit conflict even if you don't edited those lines.