Checking out Pages for Editing: Avoiding Edit Collision
File control extends into SharePoint Designer, including the ability to check files out as part of the editing experience. By default, the Require Check-out option is disabled on Wiki Page libraries. If you intend to enforce document check-out prior to editing then you need to enable this setting.
The setting for enforcing the check-out option must be done via the Web interface; it is not one of the available library configuration options in SharePoint Designer. To access and enable the Require Check-out option, go to the library’s List Settings page in the Web interface and select Versioning Settings.
If a Wiki page, or other library, has the Require Check-out option enabled then ...