On the Triad's robes, the pentagrams start with one point up, then have two points up later and by the end they are back to one point up.
When Piper is in the parking lot her car is parked in a space. After she barely backs her car up, she hits Greg's car directly behind her and both cars are in the main aisle, no longer near a parking space.
Christy Jenkins, Billie's sister, has grown up from about an 11 year old girl to a women of around 25 years old between episode 14 and 15. The age difference between the two actresses (Stephanie Patton and Marnette Patterson) is 13 years. But the episode where she was 11 was just a vision, not what Christy would look like now.
Simon tells Henry that it is destiny to combine the Marks and Matthews magical line. There is no Matthews magical line, since that is Paige's mortal adoptive parents' last name. Her magic comes from the Halliwell line.