I am seeing different behaviors for my Visualforce page depending on whether the user who runs it has development enabled or not.

I have two users that have the same role and profile but one has development mode turned on and the other does not.

I have a layout for a custom object that includes a Visualforce page as part of the standard layout.  This Visualforce page has an Action button.  The action button performs some logic and then returns a PageReference of the original page with redirect set to true so that the whole page refreshes.

When I click the action button as the user who has development enabled, everything works as I expect it to.  When I click the action button as the user who does not have development enabled, the refreshed page is display in the area that used to occupied by the Visualforce page.  The entire Salesforce page is now displayed inside the original page.

Has anyone else seen anything like this?  I have read some posts about the Visualforce page and the main page running in different domains and that makes sense - except in this case, if I have development enabled everything works.

I also see this same behavior (Salesforce page displayed within the Salesforce page) if I have an unhandled exception in the controller extension for my Visualforce page.  In this case, it doesn't matter whether I am running as a user with development privileges or not.