The scene where Homer was pressed into FBI service was taken directly from the film JFK.
Similarly, Agent Johnson, one of the FBI Agents who placed Homer on the job, was named after one of the FBI Agents from the film Die Hard.
When questioned by Kent Brockman in regards to why he procrastinated in regards to the Tax deadline, Otto remarks that he thought that the tax-line was actually the line for a Metallica concert.
Mr. Burns claims that he owns the suit that Charlie Chaplin was buried in, a reference to the fact that Charlie Chaplin's body was stolen from a cemetery in Switzerland.
When Castro remarks that Americans aren't all that bad as they named a street after him, he reacts in shock and horror when he learns just who dwells on that street, a reference to Castro Street in San Francisco, a street that is a gay community (although its true namesake was Joaquin Isidro de Castro, one of the founders of San Francisco).
One of the signs in Cuba was based on a Che Guevara poster, altered to say "Duff O Muerto!" The phrase translates to "Duff or Die!"
The "to do" pile next to Homer didn't appear until he mentioned it.
Flanders was able to send out his taxes on New Year's Day, despite the fact that post offices are closed on that day (though most post offices do have it so that way the lobby where one can drop off mail is open, but the mail won't be sent until the next business day).
When Kent Brockman begins reporting on Tax Day, a man in the background has stubble that repeatedly disappears.
Even if Burns did have a trillion dollar bill he would never be able to spend it because the US only made the one and it was known to have been stolen therefore would never be honored
After Kent Brockman interviews Krusty waiting in line at the Post Office, Krusty's bowtie changes color from purple to its correct shade of blue.
Flanders inserted mints into his tax package, but was able to insert it into the tax slot just fine.
Ned Flanders was fine with going to the post office in this episode, but in Hurricane Neddy, he states that he hates the post office.
Despite what Homer says, Lisa's book doesn't have pictures in it.
At the Cuban airport, the clerk which Burns, Smithers, and Homer go to has yellow skin in the faraway shot, dark yellow skin in the close-up from the back, and brown skin in the close-up from the front.
Much like in "Mr. Plow," this episode features a large denomination dollar bill that doesn't exist in real life, and wouldn't be considered legal tender as Harry Truman appears on it, and it's illegal to have money featuring a U.S. President or statesman who is still alive.