Sympathy for the Demon - S5-E7
Corrected entry: When Barbas posing as Leo walks down with Piper, she sees a memory of her as a child. When "Leo" asks what she sees she says "It's okay, I remember this". Little her in the memory was running down the stairs with a toy telling her grams that she loves it. Then her father Victor approaches and gives little Piper a hug, a demon magically appears and attacks them, little Piper screams and Victor protects her while grams says a spell. How can Piper says she remembers this if they never knew about magic at the time?
Correction: Grams has already been shown to have the power to erase memories. For example in That 70's Episode, Grams made it so that little Andy wouldn't remember being frozen by little Piper. That means that she could have made Piper not remember the demon.