The bottle of ketchup Uncle Fester drinks from is not entirely opaque. As it sits on the table (after he asks Tyler for a refill), the amount of liquid inside can be seen changing from shot-to-shot, and at no point is it empty, as it should be.
When Wednesday is in the Principal's office after discovering her room was ransacked, Principal Larissa (Gwendoline Christie) leans against the left side of the fireplace, but when the vantage point changes to looking out of the fireplace at her, she appears on the wrong side of the fireplace. When the vantage point returns to looking at her from Wednesday's perspective, she now appears leaning on the right-hand side of the fireplace without any indication that she moved.
When Principal Weems is speaking to Wednesday alone in her office, at one point she leans against the left side of the fireplace. When the shot changes, she is suddenly on the right side.