I'll focus on the some of ones I was wrong about:
Wasn't even that early in his career, but I wrote off Kasper Schmeichel quite early. Both when he joined Leeds (didn't watch any of their games tho), and when he joined Leicester. He had been on loan for ages, and dumped down the leagues when he left City. Thought he was signed mainly for his name, but turned out to be a brilliant signing for Leicester in the end.
Gabigol after his intial time at Inter, and failure at Benfica, but then he found form again with Santos, then came to Flamengo. Now he's one of my favorite players to watch. Think he'd do better in Europe now then when he was young.
Ron-Robert Zieler. Thought he was a dud once he left for Germany, and started playing for Hannover's 2nd team. He had previously had a loan at Northampton and hardly played, so thought he'd disappear down the divisions, but he went on to make a pretty good career for himself in Germany, and even played for the national team.
Probably lots more.