Factual error: During Tony's assassination attempt he struggles with one of the "hit men" for control of a pistol. One can see that the pistol does not contain a magazine.This would limit the shooter to 1 shot, providing the pistol is not equipped with a "magazine disconnect" if it is than it can not be fired at all. It seems unlikely that a "hit man" would be so ill prepared.
Revealing mistake: When Tony buys the orange juice off the street vendor (just before he is attacked by the hitmen) you can see the magazines on the vendor's stand being blown around by the "heavy winds" that are present throughout the episode. However, if you look behind Tony, you can see the scenery is almost completely still. The "wind" is coming from just off camera. (00:27:17)
Answer: That's not Jimmy, just a look-a-like. Jimmy's short and stout, the guy in "Christopher" episode is tall and medium built. The hair is also different.