Plot hole: Inmediately after Jamie Foxx finds the bomb in the city hall, and he says, "We don't tell the mayor anything", we see Gerard Butler arriving to his property next to the prison, and finally he enters his jail cell. So, in the time between Gerard Butler's arrival to the property and his entrance to the jail cell, Jamie Foxx thought about a plan, picked up the bomb, passed through the traffic and security checkpoints, talked to the warden to get access to the prison, entered solitary, handcuffed the bomb, and still had time to wait for Gerard Butler's arrival.
Character mistake: In the court scene Clyde mentions a case, Day v McDonough docket number 04-1325. This is a real case, but the docket number is 04-1324.
Factual error: In the scene when the police are heading to Clyde's house to arrest him, this tactic is highly unrealistic. In real life, when a person is simply a suspect in an investigation, the police do not all drive to the suspect location with lights and sirens going. This loses the element of surprise. They didn't even know if he was home, anyway.
Continuity mistake: When Darby is running from the police and heading for the police car you can see the exhaust emitting from the police cruiser tail pipe. When he gets in the car he says start the car and you can see Clyde turn the ignition/keys and the engine starts of an already idling vehicle. (00:20:35)
Factual error: When Clyde is first arrested, he is taken to a prison and put in with the general prison population. People are taken to jail before going to prison. Only after you're convicted do you go to prison. In fact, he wasn't even charged with a crime when taken there.
Plot hole: When Clyde leaves the building that he has planted the bomb to kill the Mayor and the rest of her party, he goes to his van, watches a TV feed of the meeting, then puts the van into gear and moves off. Only problem is, he didn't actually switch on the ignition, and no, Clyde didn't leave the van left switched on either, he's meticulously planned the whole operation, and isn't likely to chance his one means of escape by allowing it to be so easily stolen, and any suggestion that he'd be able to leave the van running with 'conspicuous police security' is a nonsense, as that would draw more attention to the van, something Clyde SPECIFICALLY would not want.
Character mistake: Clyde enters his cell in the final scene, and is surprised to find that he has been found out. His secret tunnel, if we go according to the characteristics accredited to his character throughout the movie, is surprisingly without any alarms of any sort. Very strange if we are to believe he is as deadly as credited. He ends dying a very silly death.
Continuity mistake: In the scene where Nick is flown by helicopter to find Mr Reynolds, the helicopter changes registration from N94PD in flight to N95PD when they land.
Factual error: The co-ordinates given by Clyde Shelton: 39° 57' 4"N 75° 10' 22W as the place to find Mr. Reynolds, is the corner of 19th and Sansom Street in Center City (1919 Sansom, to be exact, which is an apartment complex), the downtown area of Philadelphia. The scene actually takes place underneath of the George C. Platt Memorial Bridge (39° 53' 49"N 75° 12' 46W), about 4.3 miles southwest of the coordinates.
Other mistake: After killing his inmate, notice the blood on the inmate's neck is present in great amount. After camera changes, the blood is only on the shirt, the neck is clean with no holes nor coagulated blood. (00:51:00 - 00:51:45)
Factual error: In the cemetery scene when the SUVs are leaving and the EMP knocks out electrical systems, the next thing you hear is walkie-talkie communucations between the vehicles. Those would've been knocked out, as well.
Factual error: During the lethal-injection scene, an executioner is plainly visible to the witnesses through a broad glass window in the death chamber. He is shown flipping toggle switches in-sequence to release the sodium thiopental, pancuronium bromide and potassium chloride that comprise the lethal drug cocktail. For one thing, in modern Western culture, an executioner is never visible to the witnesses. For another thing, there is no "executioner," per se, flipping toggle switches in the lethal-injection process. The component drugs are loaded into the death machine in advance, and it automatically releases the drugs via a pre-set timer.
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)
Other mistake: When Clyde is in the van driving back to the prison he encounters a road block. He evades this by driving down into the parking garage. There's no way that the police or SWAT would have missed this loophole. (01:32:34)
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.
Visible crew/equipment: This is only visible in the unrated version and not the theatrical version due to different shots. When Clyde is stabbing his cellmate, a crew member is visible in the left side of the screen for a second.
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 ★