Audio problem: As Shelton is about to drive away from City Hall, we see him operate a lever on the steering column of the van and we hear the familiar sounds of a transmission shifter clunking through a few settings into "Drive". The problem is that he used the lever on the left side of the column, which is the turn signal lever. The transmission shift lever is on the right side of the column. In reality, Butler used his left turn signal before steering left into the travel lane, and the sound editors added the shifter sound to try and cover the fact he's never actually seen putting the van into gear in the scene. (01:31:50)
Audio problem: When Nick and the others are flying in a helicopter to find the buried lawyer, not only does the registration number of the helicopter change, but the model is a Bell 206 L-4. As it flies along, you hear the sound of the chopper going by. The sound of the helicopter dubbed in is of an American Eurocopter aircraft with a Fenestron instead of a tail rotor. These two aircraft have a completely different sound.
Answer: To make sure he was placed in solitary confinement. The warehouse that he owned and operated out of that was next to the prison also had a tunnel connected to every cell in the solitary wing. Clyde needed to be in one of the solitary cells so he could leave the prison whenever he needed to unnoticed, which also served to make it look like he had an accomplice on the outside.
Phaneron ★
When Nick is talking to a spook later in the movie, he is quoted as telling Nick: "That cell-mate that he killed, you think that was random? No. That's a pawn being moved off the board. Anyone who had anything to do with that case, he's gonna be coming after you." Just as all deaths played roles in Clydes game, as the audience we are led to believe this inmate played a role, but were never given any resolution as to what significance it was. Not a big deal in grand scheme of things, but unexplained.
I don't know if you just didn't read the answer thoroughly or if you didn't pay close attention to the movie, but Clyde killing his cellmate was far from being unexplained. He can't leave the prison if he's in a regular cell with the general population, so he kills the cellmate in order to get placed in the solitary wing, because every solitary cell is connected to the tunnel in his warehouse that is next to the prison, which allows him to leave whenever he needs to.
Phaneron ★