I've come across this issue a couple of times. In one instance, an OnDemand customer has imported a space but the importer didn't strip out the space layout and so he ended up with text at the top of each page in the space saying "#usernavbar()". I had to manually go to /spaces/resetdecorator.action?decoratorName=decorators/main.vmd&key=<space key>&atl_token=<token> to reset the decorator, as trying to view it at /spaces/listdecorators.action?key=<space key> just gave a message saying "You can not customise the site look and feel while a theme is active. You must first disable the currently configured theme."

The other one was an issue where the global layout had been changed in a previous version on Confluence, and upon updating to 4.1 the "Add Space" link on the Dashboard was missing. I had to manually reset the decorator by going to /admin/resetdecorator.action?decoratorName=decorators/global.vmd&alt_token=<token> to reset it.