When Holly and Sookie are discussing Halloween, Holly pronounces Samhain incorrectly.
She says "sam-a-hain". The correct pronunciation is "sah-win".
This is unlikely to be an error a true Wiccan would make.
When Bill is Talking to Nan, you can clearly see an ear-ring hole in his right earlobe. Bill being a vampire, any puncture like that should have healed immediately.