Did you have a mixed revision working copy?
I have seen this type of issue just creating tags using svn directly.
If you modify files locally, commit just those files and then do an svn cp without doing an svn update first, then you can end up with the above issue.
If this is a case of a mixed revision working copy, then there may not be an easy way to fix this, we could force an svn update, but that might be counter to what the user is trying to do, e.g. they may want to branch from their working copy... which has some files updated to HEAD while others have not been updated (because the update would break the code)