Just a short post following up on both Error: Cannot Complete This Action and Error: The website declined to show this webpage HTTP 403.
When adding web parts on a web part page, users should be encouraged to use unique and descriptive names for the web parts. (Such encouragement shouldn’t leave bruises where they would show.)
When encountering a problem with a web part that causes a need to go to the Web Part Page Maintenance page, it helps to be able to ask “Which web part did you add just before getting the error?” and get a title that is not one of five identically titled web parts.
Unique names, please!