Continuity mistake: Tony makes a toast at the dinner table in front of Carmella and Anthony. Watch Tony's wine glass during the toast, the amount of wine in the glass changes dramatically from shot to shot. (00:21:45)
Revealing mistake: Season 6, Ep. 85, "The Blue Comet": The Ukranian woman's body by the stairs keeps changing positions all the time (depending on the angle), despite her being dead.
Continuity mistake: Valentina is sitting and reading a magazine when Tony enters her hospital room. She places the magazine on her lap. Throughout the scene the magazine appears on her lap when the camera is on Tony and disappears when the camera is on Valentina. (00:18:57)
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.