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 ad 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's running, he would past the living room and kitchen.
This episode first aired on Norwegian Constitution Day, which would explain the heavy Norwegian references in the episode (coincidentally, Matt Groening is part-Norwegian).
This is the last episode where the Simpsons have a standard fullscreen television set. Starting with the 21st broadcast season and onward, the Simpsons would have a high-definition widescreen television set, while the standard definition CRT TV was there for broadcast Seasons 1-20.