Continuity mistake: When "Mulva" is giving Jerry a neck massage, check out the bathroom door in the background. It keeps changing from open to closed.
Audio problem: As the doctor is explaining how he thinks there were other factors at play to stave off the infection, when he says "something perhaps..." his mouth/lips do not move.
Deliberate mistake: When George is watching Home Alone, the sound effects and music that are heard aren't actually in the film.
Continuity mistake: At the beginning of the episode, the bag on the counter keeps moving and disappearing, and reappearing.
Chosen answer: George does mention his brother in the episode "The Parking Space". They actually really never mention anything that contradicts the fact he had a brother though, it just isn't mentioned.