Automatically activating the designer
While customizing a SharePoint form, you may add an invalid script or perform some other customization that throws an error before the form completes loading, not showing the form toolbar and making it impossible to fix the issue.
Additionally, in version 2.3, a new feature has been introduced that allows the user designing the form to hide the toolbar completely (since there are other ways to Save, Delete and Cancel).
In order to show the designer, add an extra parameter in the querystring of the page : "_mode=design" (without the quotes).
This will bring up the designer skipping the display of the customized form.
Just remove it after saving the customization in order to display the form again.