The mechanism in Confluence for creating custom decorators makes for a very brittle upgrade process. To take the example of a custom page decorator. You take a copy of the current page.vmd and paste it in to the Custom layout screen where you can then customize it.

After a Confluence upgrade, if the page.vmd has changed in the new version, there's a good chance that some page related operations will now be broken as the customised page.vmd stays in the Db.

At the very least we could flag during the upgrade process that there are customisations that should be manually checked.