Fake Nationality: Every rapper so far has been American, playing British guys, Germans, Italians, the list goes on.
Mike Diva (Bruce Lee's actor in Bruce Lee VS Clint Eastwood) lampshades this by stating that he sucks at Asian accents, so Lloyd and Peter had to coach him. Pete then proceeds to walk by and claim no involvement in said accent coaching.
An example that doesn't extend to rappers. Pew Die Pie cameos as the Russian Mikhail Barishnikov, though he is actually Swedish.
Meaningful Release Date: Rasputin vs. Stalin was released on April 22, Vladimir Lenin's birthday. It's especially appropriate, since Lenin himself has a speaking role.
Gandalf vs. Dumbledore was released on July 14, 2011, a day before the theatrical release of Harry Potter and the Deathly Hallows: Part 2.
Meme Acknowledgment: Only one day after the release of Donald Trump vs. Ebeneezer Scrooge, Donald Trump tweeted about the video, understandably confused and asking "Whats up with this rap stuff with me and Ebeneezer Scrooge?" ERB wasted no time in acknowledging that Trump acknowledged ERB.
Old Shame: While he isn't upset with how the battles turned out, EpicLLOYD understandably felt uncomfortable with playing Adolf Hitler.
Throw It In: They made a demo for the Napoleon VS Napoleon battle that eventually got redone, but they used the original for the behind the scenes video for the same battle.
In Genghis Khan VS The Easter Bunny, there's a part where Lloyd (playing Khan) smashes a chocolate egg with his sword. The behind the scenes video shows that they didn't think the idea would actually work, so they only did one try before moving on. Since Lloyd actually hit the egg on his first try (which people were not expecting), they decided to put it in.
Torch the Franchise and Run: Adolf Hitler VS Darth Vader 3 evidently will be the last match between those characters. The reason for this is clear in-universe: Darth Vader cuts Adolf Hitler in half, and it's implied that his remains land in the Sarlaac pit. Aside from story purposes, there's also the inclusion of a comment in the video that states that the match-ups between these characters are getting stale, which would imply that the crew behind the show are getting a little sick of it.
Christopher Columbus' original opponent was Neil Armstrong.
The lyrics for Bill Gates vs. Steve Jobs match originally included a line about how Steve's ailment was the only PC he couldn't beat. They decided to not use it because it didn't flow as well and crossed well into Too Soon territory.
Walked into the record industry and killed it
Got fired from my own company, rebuilt it
My legacy will never fall
But a PC got me after all
There was a battle between Hillary Clinton and King Henry VIII in the works, but it got canned due to dissatisfaction with the recording track.
Mikhail Gorbachev's iconic crimson birthmark was originally going to spell out "ERB". Lloyd didn't really like the idea - mostly because it was impossible to see without tilting his head absurdly - so they settled with the birthmark's actual look.
Maximillian of Assist Me made a parody of this series called "Epic Rap Battles of MvC3" between Albert Wesker and Doctor Doom. He was going to make more, but the Epic Rap Battle people sent him a Cease And Desist, basically saying "this one's okay, but if you make more, you'll hear from our lawyers." (No, they don't mean Phoenix Wright...)
The BTS for Miley Cyrus vs Joan of Arc shows a hanging rope prop that most likely was intended for a "Wrecking Ball" reference. They also went over possible lines for Peter and Lloyd's cameo as Miley and Lilly. Neither of these are in the final cut.
Blackbeard's other opponents considered for the battle were Jack Sparrow and Cap'n Crunch before deciding on Al Capone, who, possibly intentionally, references both Jack and Cap'n in his first two lines.
Word of God: Peter makes a comment about Hitler and Vader never being able to settle their differences in the Season 3 Opener behind-the-scenes video, so there's a likely possibility that they will show up yet again if/when season 4 comes out.
They've confirmed that they're not interested in making Homer Simpson versus Peter Griffin as it just sounds too boring. Similarly, they won't do Tobuscus versus Pewdiepie, as they love both of them and are unsure as to how it would work. Many times, a rap battle between Obama and Osama bin Laden has also been ruled out, though Obama later appeared rapping against Mitt Romney.