Broken brake return spring.
From http://picasaweb.google.com/thisnthatntother/Fail?authkey=8ePsxVY29kw# as of 2008/12.
Caption says
Avid BB7 Curly spring. snapped after 4500 hard off road miles over 3 years. Brake was still usable but didn't return so pads dragged a lot.
See also FAIL-159.html