A previous question/answer here mentioned Mozart's ability (when he was a teenager) to sit and listen to a performance of Allegri's Miserere, remember most of the details, and then write it down later that day.
Is this really possible? Even for a genius like Mozart, all the different voices, the length of the piece, and the fact that he only heard it once—not to mention his age—makes it seem a bit unrealistic that he could remember, let's say 75%, and then write that down from memory with less than, let's say 25%, of error.
Is this really possible? Or it is likely that someone with him (his father) helped also, or they were writing it down as they were listening? Even if he had help or was able to transcribe while listening, it still seems unrealistic.
Assuming this is possible, how exactly does one achieve this ability (even remotely close)? I know ear training is a huge thing, and that with time and practice you will be able to dictate, however how can one listen to an extended piece of music (more than 10 minutes) and be able to transcribe all voices/instruments in a manner that is at least, let's say, 75% accurate?
Wiki page here
EDIT: Thanks everyone for the great discussion.