When just Leonard and Leslie go back over the section together, Leonard draws the bow over the cello the correct way when he plays a note to tune it. But when he actually starts playing the part with Leslie, he has flipped the bow around so that the wooden side is touching the strings. Since Johnny Galecki knows how to play the cello, he might have done this intentionally to avoid playing any real sound causing a conflict when they added the music in post. When he does his tuning note, he can then be seen twisting it upside down for the fake playing.
Howard talks of Leonard and Leslie making eine kleine bang bang music. "Eine Kleine Nachtmusik" is Mozart's most popular piece for string quartet, although today it is often performed by a string orchestra.