Phoebe claims that maybe Chris's conception wasn't meant to be and that it is possible that his ultimate destiny was to warn the sisters about Wyatt. If Chris wasn't conceived, though, then he would have never been able to travel to the past to warn them, a fact he states earlier in the episode.
When Paige leaves Piper, Leo and Wyatt to go to the kitchen to talk to Phoebe, the styling of her hair has changed between those two shots.
When Gideon tells Sarah the student to be careful with the school books, she has her arms down by her sides. In the next shot, Sarah has a whole armful of books, with no time to have picked them up. She is picking up more books by hand at normal speed, so this cannot be explained away as 'magic'.
Piper, Leo, Paige, and Phoebe take Piper's car to the alley to fight the dark lighters. After Phoebe is hurt, Paige drives her to the hospital. After the hospital, Paige and Phoebe orb back to the manor, when they should have driven again.
When Phoebe falls off a building, she gets dirt all over her jacket and a bleeding wound on her shoulder. But when Phoebe and Paige return to the manor after visiting a hospital, Phoebe's bandaged wound is at the back of her neck, not her shoulder, and there is no blood or dirt on her jacket.
When the dark lighter attacks at the manor, Paige calls for his arrow, but the arrow is already shown orbing before she calls for it.
At the end when Piper and Leo are brought back to the real world and the sisters asked the dark lighter who sent him, he is shot by another dark lighter's arrow before he could answer. The arrow hits him from straight in front but when they locate the culprit he is on the roof over the dark lighter's right shoulder. The arrow should have hit him in the back.