Originally reported internally as CONFDEV-22240, which has some discussion on it.

Steps to reproduce:

1. User A starts editing a page and makes changes
2. User B starts editing a page and makes different changes
3. Both users click "Save" at about the same time, such that the database transactions with the changes are running concurrently.

Expected behaviour:

• Assuming the change by user A commits first, user B is sent back to the editor and notified that their changes conflict with changes made by user A. Or, if the changes are compatible, user B's changes are included after user A's.
• One copy of the previous content is added as a historical version.
• If the changes do not conflict, then a subsequent historical version is added for user A's changes.

Actual behaviour:

• Two copies of the old content are added to the change history in the database, but because they have the same version number only one will ever be shown.
• User A's change is completely lost. It is not in the change history and is not shown in the content.
• User A is not notified that their change was lost.

Workaround:

• None.