Polish fans were throwing a lot of jokes about the words "Kaczyński unit" from the first episode, because of Polish president at the time. This stopped being funny after said president and several members of parliament perished in a plane crash.
Britannia not being part of the EU has become this ever since the 2016 referendum in Britain that resulted in its secession from the real-life EU. This is one origin of the "Nigel Farage makes anime real" meme.
Look closely enough in episode 22, and you can see a map where it is clearly shown that Great Britain is a neutral nation and not affiliated with neither the EU nor Britannia.
This wouldn't be the last time a character voiced by Yuri Lowenthal would have to tragically spend the last moments together with a loved one while she was on her death bed. Worse, in both cases the death was caused in some way by someone Lowenthal's character once trusted. Worse still, unlike here where he does eventually patch things up with the person he once trusted, in the other case he permanently remains enemies with the other character.
At one point, Luciano Bradley implies he wants to rape Kallen. Bradley's English voice actor, Vic Mignogna, would later be accused of sexual misconduct.
In Episode 19, Kallen claims that Euphemia is a useless puppet princess of Britannia who can't make any decisions of her own. Some episodes later, Euphie gets killed after being accidentally mind controlled by Lelouch (himself a rebellious member of the Britannian nobility) and stripped of her free will.
Kallen does NOT take well when she learns Zero's true identity and hear him claim that he'd been using Kallen herself as his puppet. She does realize this is all part of his facade and accepts him as Zero, going so far as to be the only one to defend him when practically everyone else has turned on him due to Schneizel.
Shirley’s father’s death was already a Tear Jerker. But seeing Shirley attending poor Joseph’s funeral becomes even worse when Shirley herself dies just one year later.
Britannia is not part of the European Union, and as of 2016, neither is the real-life United Kingdom.
Villetta said she would kill Shirley a thousand times after she forced Villetta to wear a seriously skimpy bikini for the swim club. In episode 13 of R2 the advice she gives Rolo accidentally helps cause Shirley's death. Doubles as Harsher in Hindsight considering Villetta using Shirley a season earlier really helped get the ball rolling there.
Lelouch's extremely fake-looking sword, which Suzaku as Zero uses to impale him in the finale.
After causing an avalanche at Narita, Lelouch jokes that he should have consulted Nina about this. This has a relatively apparent and obvious FAM a few episodes later, when Shirley's father died in the avalanche as an unintended victim of Lelouch's plan, and a more subtle one later when Nina's scientific expertise creates a bomb that kills millions of people.
When explaining their history together, C.C. says she left Mao because he was unable to complete their contract when the time came. Once we learn midway through the second season that her wish is to die, the fact that Mao was abandoned after having his life ruined by Geass simply because he loved her too much to either kill her or let her die becomes this.
. This stopped being funny after said president and several members of parliament perished in a plane crash