The foot pedals of Bugs Bunny's concert grand piano appear in different configurations throughout the short - a visual gaffe that, in a narrative sense, doesn't really matter because Bugs never uses the pedals. Director Friz Freleng took plenty of liberties with real piano playing just to get the gags over.
When Bugs is shielding the keys he's playing, the mouse is nestled between two black keys. The spacing of the visible black keys is not correct - all 5 are separated by 2 white keys. Other times though, such as when the mouse is running along the keyboard or when they are playing the duet, the keys are depicted correctly.