Plot hole: According to the family tree viewed in this episode, Penny "Grams" Halliwell was born on 6/23/1937, and her daughter Patty, the Charmed Ones' mother, was born on 4/5/1950. In addition, Patty's father was born in 1930. So, according to this, Penny gave birth to her daughter at age 12, while married to a 20 year old man. That's quite outlandish, to say the least.
Continuity mistake: On 2/17/1999, Phoebe realizes that her past self, P. Russell, died on that same date in 1924, and was the same age as Phoebe is now. According to the family tree she is looking at, P. Russell was born on 7/2/1894, meaning that she was 29 upon her death. But Phoebe was born on 11/2/1975, according to that same family tree, making her only 23 at the time of this episode.