Big Cass. WWE cared about him way more than they should have, and he wasted it by not following directions. In addition to that, he alienated people in the locker room because of his
opinions. If he managed to stay, chances are, he would be a former Universal Champion by now.
There are also people who didn't fumble the bag permanently, but they set themselves back for a while.
-Jeff Hardy in 2008. He was supposed to win MITB that year, but he got suspended for drugs and his main event push didn't come until later in the year.
-Matt Hardy in 2005. He was the hottest he had ever been in his career and the fans were 100% ready to get behind him in his feud with Edge. But he was only used to get Edge over as a main eventer, and he got killed on the mic as well. Edge responding to the "car accident" promo had to have been him shooting somewhat.
-Jinder Mahal. Triple H said something about him being immature and not working hard enough the first time he was in the WWE which is what led to his release. He busted his ass to get back, cut out drinking, built up muscle, and became the WWE Champion.
-Drew McIntyre had the same issues as Jinder, I think.