Corrected entry: In the episode "P3 H20" we learn that Patty died because of the Water Demon, which is why Prue is afraid of water. However in this episode we are told that Patty was killed by the Sea Hag, and that's why Piper develops her demon fear upon becoming pregnant.
Corrected entry: Leo tells Piper that "there was no one there to finish [her baby book] after [her] mother died," and that's why every page from "The Sixth Year" onwards is blank. But Piper and her sisters were raised by Grams after Patty died. Why wouldn't Grams have continued filling in the baby book?
Correction: Maybe seeing the baby book that Patti started and never got a chance to finish was too painful for her. It's not really a mistake, it's impossible to predict how grief will affect people.
Correction: Paige actually mentions the water demon. Saying that the Sea Hag reminds her of it and that it kills in a fascinating way. That is when Piper states that she knows about the water demon because it killed their mother.