When Gideon is eating his take out in Garcia's office, Penelope comes in and closes the door. When it is time for her close up she is standing with the door open again.
Will sings the song "This Is My Father's World" with his music teacher at the beginning of the episode. The sheet music is displayed both on the piano and later spattered with the victim's blood. Although the sheet is titled correctly, the music on the sheet is not "This Is My Father's World" but is instead a beginning piano piece called "The Spinning Wheel."
In the Prologue, Wally, the boy, has a beige square in his right ear (perhaps a device to assist the actor with his lines, with a tiny antenna), then the next shot it's gone, then it reappears in the next shot.
Music teacher Annie Stuart is clearly faking playing the piano.