In git it is possible to tag a commit that is on a branch, but then move or delete the branch head so that no branch references a descendent of the tag.

This can also happen as a result of converting from SVN when the svn repository has complex tags (see http://svnbook.red-bean.com/en/1.6/svn.branchmerge.tags.html#svn.branchmerge.tags.mkcomplex) or tags that are modified.