Throughout the years, numerous characters have referred to "the city" (meaning Sydney) as being either "up" or "down" from Summer Bay. The general consensus, however, is that Summer Bay is north of Sydney (approximately 100km away), as is the real-life filming location.
When Carly Lucini had her baby in 1992, it was a girl. But when the Ross family tried to help Pippa regain her memory in 1997, they said Carly had had a boy!
Due to being a long running soap opera characters history's and backstories are retconned and certain plot points are forgotten i.e. characters celebrating their birthdays on the wrong days or forgetting about key details in their lives.
The original premise of the show was that Tom and Pippa Fletcher had begun fostering children because Pippa could not conceive a child - therefore when she discovers she is pregnant in the pilot, it is considered a miracle. However, Pippa becomes pregnant twice again, with no mention made of the unlikelihood of this.