Atop of the piano, Batman is standing to the Joker's left, and Robin to his right. The goon throws Robin onto the strings almost parallel to the keys, beyond that point where Batman had stood. However, as they land on the strings, Batman is still on the left.
During the giant piano sequence, the intervals between the keys the Joker lands on do not match the direction of the hammers inside the piano. For example, one sequence where the Joker "plays" a descending scale, but the hammers hit what should be an ascending scale.