When trying to delete a content type from SharePoint Designer you might run into following funny error message:
Could not save changes. The Content type is in use.
You need to check:
- This content type is not being referenced by a list or document library
- This content type is not being referenced by a list or document library that’s in End User Recycle bin
- This content type is not being referenced by a list or document library that’s in Site Collection Recycle bin
- If content type was deployed via feature you need to retract it