Witchstock - S6-E11
Plot hole: In the opening scene, Chris directs Piper and Phoebe on how to vanquish the amorphous demon. He says that it will attack "fast and hard" as soon as it senses Phoebe's magic when she levitates. But Chris had orbed out of the warehouse and back in, and the demon didn't react to the magic. Later in the episode, it is specifically attracted to a use of orbing, several times.
Witchstock - S6-E11
Plot hole: This episode takes place in 1967, and Penny (Grams) is married to a man named Allen Halliwell who dies in the episode. Allen is supposed to be Penny's "first husband," Patty's father and the Charmed Ones' grandfather. But it was previously established that Patty's father was named Jack, and he died in 1964. Also they hint that Patty is still little but we know she was born in 1950 so she would be 17.
Witchstock - S6-E11
Plot hole: After Paige changes the past, the ghost of Grams changes from a no-nonsense demon-hunter into a pacifist hippie. So, apparently, we are now in an alternate timeline where Grams never became assertive, and stayed a hippie all her life. The episode went to great lengths to establish that if Grams had stayed a hippie, she and the Charmed Ones would have all died young. But the ghost of Grams was still middle-aged, instead of decades younger, and Wyatt and Chris were still there, but they wouldn't have been if their mother Piper had died as a child.
Answer: "Hush, Hush, Hush" Paula Cole.