This page is about the technical problem where multiple editors try to edit the same page at once, leading to a conflict the software cannot resolve automatically. It is not to be confused with edit warring.
This page in a nutshell: Edit conflicts occur when two users make edits to the same page at the same time, causing the edits to conflict with each other. When this happens, the MediaWiki software provides a screen that allows the user to resolve the conflict manually.
This page discusses edit conflicts, and how to deal with them. To understand what an edit conflict is, consider the following situation:
Bob clicks "Edit" on a page. The software sends Bob the current revision of the page, #1.
Alice clicks "Edit" on the same page, while Bob is editing. The software sends Alice the current revision of the page, #1.
Bob finishes his edits and clicks "Publish changes". The software saves Bob's edits as revision #2, and publishes #2. Alice is still editing #1.
Alice finishes her edits and clicks "Publish changes". The software saves Alice's edits as revision #3, but discovers that #3 is based on revision #1 instead of the currently published revision #2. The software can sometimes automatically reconcile the differences; if it is unable to do so, Alice will see an "edit conflict" page, giving Alice a chance to reconcile the differences between #2 and #3 manually.