A shielding curse was a form of magic related to shielding blessings. They could indeed be caused by the former going wrong, as in the case of Eragon and Elva. Eragon attempt to bless Elva failed due to his poor grasp of the grammatical structure of the Ancient Language. Whilst she should have been shielded from misfortune, the "blessing" made her a shield from misfortune, binding her unwillingly to a fate she did not desire. Shielding curses can be removed, but not easily or, as with Elva, entirely.