The title is something of a misnomer. High C (also known as C6) is a single note; nothing can be "in" it. A better title would have referenced a musical key, such as "Murder in C Minor".
Several times, Piero says he is from Zürich, Switzerland; yet he speaks Italian as a native language and bears an Italian name. Neither makes sense for people from the area around Zürich. The Swiss city is near the border of Austria and Liechtenstein and the natives speak German.
In the opening scene, the "cobblestone" street flexes as someone steps on it.
The chalk drawing is perpendicular to the way the dead body was found.
The episode takes place in Genoa. In one of the early scenic transitions they show the opera house which is supposed to be where the action takes place , but it's an image of La Scala, which is in Milan.
When Rossini and Faber are arguing in front of the mirror (and Faber is wearing the operatic costume and holding the revolver), Rossini refers to Faber as Petrocelli.
Jessica has in the past suggested a Gunshot Residue test. Since the accused was standing over the body holding a gun and saying they did not fire it, it should have been performed.