There was one that I can think of reading about and doggone if I can remember his name. I believe he was in the very low weight classes. hh will know.
hh actually posed this question in a thread of his about 6 months ago... and the answer was: Francisco Quiroz and Aquilies Guzman.
Now, Quiroz was 9-10-1 when he upset and retired Lupe Madera for the WBA LFW Title in 1984. Does this make him the only successful world title challenger in history with a losing record at the time he became champion? (Remarkably, he actually made a successful defense against a credible challenger before dropping it over the championship distance to a very highly touted Joey Olivo.) Guzman was at least 6-5-1 when he won his title. Boxwreck currently indicates that he retired at 14-14-3. Neither Quiroz or Guzman appear to be the absolute worst or least qualified successful challengers in history, although their records are a matter of statistical fact. Neither seems quite the equivalent of World Championship Wrestling bestowing their title belt on David Arquette.