Sometimes, two or more people may edit a page at the same time. When this happens, Confluence will do its best to ensure that nobody's changes are lost.
How will I know if someone else is editing the same page as I am?
If another user is editing the same page as you, Confluence will display a message above your edit screen letting you know who the other user is and when the last edit was made.
Screenshot: Notification of Simultaneous Page Editing
What happens if two of us are editing the same page and the other user saves before I do?
If someone else has saved the page before you, when you click 'Save', Confluence will check if there are any conflicts between your changes and theirs. If there are no conflicting changes, Confluence will merge both the edits successfully. If there are any conflicts, Confluence will display them for you and give you the option to either 'Overwrite' the other user's changes, 'Merge your changes' manually, or 'Discard' them.
Screenshot: Notification of Page Editing Conflict
For example, Alice and Bob both edit the same page at the same time.
If Alice clicks save before Bob, Bob is now effectively editing an out-of-date version of the page. When Bob clicks save, Confluence will examine his changes to see if any overlap with Alice's. If the changes do not overlap (i.e. Alice and Bob edited different parts of the page), Bob's changes will be merged with Alice's automatically.
If Bob's changes overlap with Alice's, Confluence will display an error message to Bob showing where Alice has changed the page, and giving Bob the options to overwrite Alice's changes with his own, to re-edit the document to incorporate Alice's work, or to cancel his own changes entirely.