Revealing mistake: Jackie is shot in the head, and not only does his head not burst, but the snow mount in front is already covered all over in blood before he falls.
Visible crew/equipment: When AJ tries on his new military school uniform, he's pressured to also put on the hat. Then, after AJ looks at himself in the mirror, just as he walks back toward his mother, the reflection of the boom mic becomes visible in that mirror directly over Carmela's reflection, at the right side of the screen.
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.