Its a cycle thing isn't it? If not for the loan, the likes of Diarra wouldn't have gotten the required matches and experiences to showcase their talent or improve. It gets recycled back to the question of what the intention to loan out the player was for? To simply move him out/put him on the transfer window or in view of progress and future integration back to the team.
We tend to loan players back to their country whereas our youngsters who went out on loan seldom feature regularly or normally went to the lower leagues. Those who were loaned out usually have very short duration left on their contract too, allowing them to basically leave for nominal/no fee the season after the loan. It seemed that the moment they were loaned out, they were written off. Haven't thought of anyone who was loaned out with 'purpose' and then let go off wrongly. Thus, don't really think anyone could fall into the 'mistakenly loaned out' players category. If there really were players under such category, I'd say its the likes of Hamill, Anderson and Hobbs. Yet they haven't 'turned out to be good for another team' (of our 'size') yet. 😛