In previous versions of CCK, I've been able to create a like-named fieldset across multiple content types. A practical application of this is that I have a host of fields that represent denormalized data that I do not want to expose to the user. So, I throw them into a group called something like "Hide Me", and then have some code that hides the fieldset and child fields.
In RC10, an error is throw stating: "Add new group: The group name group_hide_me already exists." I've reviewed the data in content_group and content_group_fields and the content type I'm trying to add the group to does not have any groups associated with it.
Looking through the code, fieldgroup.module:374 loads all of the groups and then a foreach is done twice, forcing unique group names across all content types. Marked this as a bug since unique group names has not been enforced in previous releases. If this is intended, it would be helpful to update the documentation that comes with CCK to state that a group name must be unique.