Frank Grimes states that Homer has the intelligence of a six year old.
Lenny and Carl have Master's degrees, but Homer's degree, which he earned in "Homer Goes to College", isn't mentioned; Homer didn't complete a degree - Burns just had him take Nuclear Physics 101 to comply with the inspectors' requirement that Homer have college-level training in nuclear physics.
Homer shows Grimes his pictures and his Grammy, which reference numerous previous episodes:
A fourth shows Homer, Barney, Apu, and Skinner winning a Grammy award which references "Homer's Barbershop Quartet". He also shows the Grammy award to Frank.
A police auction would never let a piece of property go for one dollar. Realistically, the minimum bid would be much higher. Springfielders are not known for their intelligence.
Bart removes two of the three safety signs on the staircase in the factory. Then, the third one vanishes on its own.
Marge says that she has five lobsters for the dinner with Frank Grimes. She should only need four, because Maggie is a baby and Lisa is a vegetarian. It is possible that Homer was planning on eating two lobsters, or that Abe was invited but didn't show.
When Bart arrives at the factory after it falls over, Milhouse appears to be bald.
At one point, Sarah Wiggum is seen in the audience at the power plant model contest. Unusually, she isn't wearing lipstick.
In a far shot of the audience at the power plant model contest, the lenses of Charlie (SNPP)'s glasses are white instead of aqua.
Grimes says "D'oh" when imitating Homer. Homer never said "D'oh" in Grimes' presence. It's possible that Homer said "D'oh" in Grimes's presence off-screen.
During the funeral, the Simpsons move from the second row of seats to the front row.
The poster for the Children's Design Your Own Power Plant Contest is behind Homer when he is in the break room. Later Frank takes the poster and erase proof of it being for Children. If it was behind Homer when he was in the break room wouldn't he have not seen it and realize it was the actual poster without the evidence of it being for children?
At the contest, Chief Wiggum is heard but not seen. Even if he was, he would be sitting next to his wife Sarah who is seen in the audience.