Incorrect results may return when using the API below with the recursive property set. In this case, results from other folders may appear in the results.
ContentManager cm = new ContentManager(Ektron.Cms.Framework.ApiAccessMode.Admin);
ContentCriteria criteria = new ContentCriteria();
criteria.FolderRecursive = true;
The real cause is that there is corrupt content in the folder or one of it's sub folders. You can usually identify the errant content as it will be in a non-approved status and it will not be editable. It can also be an item that was not archived properly.