When Piper travels back in time with Grams and her mother to banish the Hollow, the banister on the staircase is destroyed when they arrive (although it's not very visible/noticeable). Somehow, it is miraculously repaired when Billie comes down the stairs right after the Hollow is banished. In the rest of the scene, it is destroyed again.
When Patty and Piper explain to Grams why they are there, she seems surprised to learn that the girls are the Charmed Ones. However, in Season 1, when the girls went back in time they told her and this scene is from a point in time after those events from the 70s. Although she did say that she would cast a memory spell to forget, in Season 2, Aunt Gail mentions that Grams told her who the girls would become, indicating that she did know.
Grams was disappointed when she learned that Paige married a mortal. She said "Didn't I teach you girls anything?" Grams' first husband, Allen, was a mortal (Patty's father). This was learned in episode 6:11 "Witchstock". Paige actually followed in her footsteps.
At the end, Piper's granddaughter shuts the door magically in the wrong direction, or so it would seem, but there's no indication that her powers don't work exactly the same and that she bounced the spell behind the door to push it from the same direction.
Grams acts like she doesn't know Leo, when in fact he attended the magical happening Grams planned "Witchstock" (season 6, episode 11). It was explained, however, that Grams, being as powerful a witch as she was, could cast a memory spell and erase the events of the time traveling in order to keep from changing the future.
Patty says she is from 1975, and it seems like Phoebe has just been conceived. In "Pardon my Past", Phoebes birthday is given on the family tree as being 11/02/1975. People could interpret this date as 11 Feb 1975, and therefore, they may claim that Paty's statement is wrong. However, the date actually means Nov 02 1975. So Patty saying she's from 1975 is correct, since Phoebe was conceived in that same year.
Grams acts as though she doesn't know that Prue, Piper and Phoebe become the Charmed Ones, or recognize "future" Piper. In fact she acts as though time travel is a big surprise. Where as Grams had meet the Charmed Ones in season 1 episode 17 "That '70's Episode" so should not have been as surprised as she was. Young Grams said in that after they leave that she was going to cast a memory spell to erase the knowledge that they had time traveled to avoid changing the future.
Near the end of the episode after everything has been fixed, Wyatt orbs next to Piper to show that he has his powers back. The footage of Wyatt orbing and Piper standing are obviously not taken at the same time as the screen in the middle of the two characters is not lined up properly and briefly pulls apart before realigning.
When further Billie comes down the stairs to join her past self the railing of the stairs is completely in tacked but before and after it is seen as broken.
Piper tells 1975 Patty that she has two more daughters, making a total of four daughters. A few minutes later, she says they need the power of three and Patty realizes that her daughters are the charmed ones. Patty doesn't question them being the charmed ones when there are four daughters.
The Chris that was "killed" at the end of Season 6 is supposed to be a Chris from an alternate time line where Wyatt turned evil. Since his parents and his aunts were able to prevent the events that made Wyatt evil he never had to go back to the past and thus survived. They act as though he is the same Chris however, asking him if he remembers where Grandpa's (Victor's) place is, and him recognizing Grams, even though Wyatt doesn't. If he is a Chris from a different reality and never came back from the future, he shouldn't know these things.