6.1.X EE, 6.2.X EE, 7.0.0 M3
1. Enable local or remote staging with page versioning
2. Add a page, add two portlets
3. Click on "History" and select a previous version
4. Add another portlet
5. Refresh the page
Result: The portlet disappeared.
If we are in the latest version of the page, modifying the page creates another version and the new version is displayed.
On the other hand, if we select a previous version of the page from the history, we are stuck on that version. After this, each modification on the page will create a new version compared to the selected version.