When Prue, Piper and Phoebe take the children away from the manor in Grams's car, the shot of them leaving the driveway reveals that there are no children (or anybody else) in the car with them.
When traveling in time Phoebe's hand moves from near her face to holding Prue's hand.
Little Andy has brown eyes but older Andy has blues eyes.
In the opening, there is a movie theater showing Jaws after Piper, Prue, and Phoebe go back to the 70's.
Jaws was released on June 20, 1975. Phoebe picks up a paper after Grams throws them out of the house and the date is March 24, 1975. Jaws would not be in the movie theater on March 24, 1975, since it wasn't released for another 3 months.
The events take place largely on March 24th, 1975. Which according to the sisters' birth dates, that would make Young Prue 4-1/2 years old (born October 28th, 1970) and Young Piper nineteen months old (born August 7th, 1973 according to the family tree). But the child actress who plays Young Piper was six years old at the time of filming this, which would make the character a very tall and very verbal 19-month old baby or toddler.
When the girls go back to their time, Nicholas is in their house in the past because he realized the ring had been stolen. There is nothing keeping him from forcing Patty to bless the ring again, or killing Penny, Patty, little Prue, and little Piper. Penny and Patty obviously didn't defeat him, since he was still in the house when the girls got back to their time.
The child that is used to play young Piper has green eyes. Holly Marie Combs' eyes are a dark brown.