When the body field is removed from the content type, and the content type's "Teaser" setting (under Workflow) is set to "Manual excerpt", Excerpt creates an additional spurious "Input Format" fieldset that breaks WYSIWYG editors that follow it.

This stems from excerpt_form_alter (excerpt.module) where it checks whether the setting is switched on, but not if the body field is actually present. The form_alter modifications must push in the Body field input format where it's not actually used.

It doesn't really make sense for Excerpt to be set to "Manually generated" if there's no body field, and arguably this is the user's responsibility. However, it's not that obvious and it would be helpful to automatically switch off the "Manual excerpt" setting if there's no Body field on the content type.

The symptoms of the problem are similar http://drupal.org/node/1040370.