MediaWiki:Explainconflict

GAH! Someone else has been editing this page since you started editing, causing the dreaded EDIT CONFLICT.

The other guy's version of the article is in the top box. Your version of the article is in the bottom box.

You will have to merge your edits into the top box.

When you press save, ONLY the version in the top box will be saved. The bottom box will be  lost     forever....