Like someone else mentioned, Yordanis Despaigne. I watched his fight with White, and in the first two rounds, he was schooling White, but he had this lack of focus about his body language and then all the sudden he was getting hit left and right and seemed not to care. Despaigne, although never scored an Olympic medal, he dominated Pascal in the amateurs, went 1-1 with Dirrell, and had a close back and forth brawl with Korobov, and Korobov later said that Despaigne was his toughest fight ever. I think Despaigne could still go far if he stays focused.
Speaking of Korobov. This guy looked like a powerhouse when he debuted as a professional, and all the sudden he started looking average, and even somewhat tentative and also losing rounds against guys that he should be blowing out inside 4 rounds. I have a feeling when Korobov starts to really step up, we're going to find out he's a disappointment.
Like someone else mentioned, Henry Akinwande. This guy had it all, size, athletic ability, one punch KO power and good boxing skills. He would have these lackluster performances where he'd be very tentative, and then out of nowhere in his next fight, come in and DESTROY dudes. (Go watch his fight with Jeremy Williams). Then he fought Lewis and got DQ'd for excessive holding, what a disappointment.