Two of the four actors playing resistance fighters John Connor sends to 2007 are not the same actors killed by terminator Vic earlier in the season.
If Sarah's blood-type is O-negative, then it is impossible for her to have a son that is AB-negative. Assuming that Kyle was AB-negative like Derek, then John could only be A- or B-negative; not even O-negative. Also, either John or Sarah should have been able to give Derek her blood, as she is a near universal donor, Derek is a near universal acceptor and they all have the same Rh factor.
Charley Dixon says the bullet nicked Derek Reese's left lung but Derek got shot on the right side of the chest, not the left side.
In the first Terminator movie it is established that Kyle Reese came from the year 2029 to protect Sarah Connor from the terminator. But Cameron says Kyle Reese was declared MIA in 2027 after being sent to protect Sarah Connor.
When Derek Reese requires AB-negative blood, John asks Charlie to test his blood group. He is some 15-16 years, how come his blood group was never tested and he, or her mother, doesn't know what John's blood group is, especially because of risks to John's life that might hurt him enough to require huge volumes of blood any time.
It wouldn't have taken Derek Reese mastery over rocket science to figure out that John was his brother Kyle's son. Of course, there was additional angle that Sarah and Charlie were going to marry in the past, so one might think that John might be Charlie son, but, especially after the blood group match for which John has insisted very confidently as if he was aware that the blood group has to match. Kyle was Derek's brother so seeing Kyle so possessed by the pic of Sarah, Derek should have asked Kyle himself about it, and being brothers, Kyle is likely to have told him. Family can be expected to keep such secrets from non-family persons. Similarly Cameron with her high intellect should have figured it out just like Charlie, being a qualified doctor, figures it out on his own. Also, as the person having arrived in the past time line cannot go back to the future, so Derek is bound to remain in the current time line, so there was no possibility of Derek accidentally revealing this secret to any other member of resistance in future, so Sarah or John should have told him.