During the couch gag, after the family runs into the bar from Cheers, Sideshow Bob walks in. Sideshow Bob is voiced by Kelsey Grammer, who also portrayed Frasier Crane on Cheers.
It seems a little strange as to why all the Ogdenvillians would go to Springfield and not other neighboring cities, like North Haverbrook or Brockway (as mentioned in Marge vs. the Monorail), or better yet, move out of whatever state Springfield is in.
When Chief Wiggum is at the checkpoint, he mentions to Lou that Springfield only has three police officers. After the completion of the wall, five Springfield Police officers are shown in uniform, with an additional two in a SWAT element.
When the police arrives at the wall, Eddie was driving the police car with Lou in the passenger seat and Chief Wiggum was in the passenger seat of a different vehicle. A scene later Wiggum was in the passenger seat of Eddie's vehicle.
Also, Xylophobia is the fear of wooden objects, not of xylophones.
In the scene where Lou talks to Chief Wiggum, his mouth disappears for a frame.
In one scene, where Homer gets food poisoned; he runs to one of the bathrooms, he runs across the hallway which somehow appears in the house from nowhere. If he is running, he would past the living room and kitchen.