Buster Douglas, Frank Bruno. Tyson Fury probably falls into this bracket too, after fighting his demons etc.
You got me ! Fragile he was not, way way past his best he certainly was. I knew someone would pick me up on this !
James J. Braddock. 11-20-2 run. Serious problems with right hand. Battling poverty in the Great Depression. In 1935 beat Max Baer by UD to become World Heavyweight Champion.
Francisco Quiroz was 9-10-1 and had won 3 of his last 15 when he beat Lupe Madera for WBA light flyweight.
I like the thread idea, but; Beating Francisco Javier Prieto for a vacant title isn't much of a championship claim. Khan at least beat a worthy fighter for his title, defended it against some fierce comp (Chino), unified and took part in a bout to decide The Ring's 140lb champeen, while Wlad beat a worthy fighter for the IBF, then unified and became top dog. Linares has always been painfully overestimated. His bounceback from shattering defeat wasn't too impressive, in truth. Just a who's who of fringe level contenders. If only "title" was still a synonym for legitimate championship status. Tippy Larkin (known as much for for his vulnerability to a good shot from a puncher as for his sublime skills) could've won umpteen paper titles before losing them to a big banger in these days, but there was only one he could challenge for back in those times.