When Phoebe is watering the plants at the beginning, there is obviously no water in the can.
In the obituary page that Paige reads about Prue's death, Prue's name is spelt incorrectly. First it is spelled the correct way (Prudence and Prue), but further down it is spelt incorrectly as Pru.
Patty says that Piper and Phoebe were toddlers when she was pregnant with Paige. However, in "That 70s Episode", Piper appears to be about 4 and at that point Patty was barely pregnant with Phoebe so when Paige came along, Piper would have been about 6 and not a toddler as Patty states.
Piper doesn't light the candles the second time she summons Grams.
It was established in Season 2 that Leo has been watching over the sisters their whole lives so it seems highly unlikely for him to not have known about Paige.