Gingerbread - S3-E11
Factual error: When Giles and Cordelia are driving to save the girls from getting burned at the stake and are making the potion to make the demon show his true face Giles tells Cordelia to shred the Wolfsbane. This is a huge mistake as any Witch will know (my sister pointed this out to me) that Wolfsbane is highly poisonous and should never be touched with bare hands and Cordelia didn't have gloves or anything.