The Cat in the Hat wears sunglasses identical to Nice Peter's signature look. His voice is also the same.
May or may not have been intentional, but the Wright Bros., who launched their first plane in North Carolina, are played by Rhett & Link who were both born in said state.
NicePeter plays Albus Dumbledore in the eleventh battle, where he finishes his last verse with "Imma fly like it's Quidditch". This was the chorus of one of NicePeter's own non-rap battle songs.
The 10th Doctor calls Doc Brown "the wannabe Einstein minus the 'stache". MC Mr Napkins played both Einstein and Brown.
Bruce Lee tells Clint Eastwood that he "fucked up Chuck Norris". Both Eastwood and Norris were played by EpicLLOYD.
Moses is seen with smoke coming from his mouth, and even directly references some of Snoop Dogg's earlier lyrics in his verses.
Performers reprising characters played in their earlier works is not without precedent: Lisa Nova as Sarah Palin, NicePeter as Santa Claus (accompanied by an EpicLloyd elf), Dante Cimadamore as Nikola Tesla, and numerous videos of Alphacat as Barack Obama all predate their portrayal in the Epic Rap Battles.
Another ambiguously intentional one; Sir Issac Newton, played by "Weird Al" Yankovic in his second verse tells Bill Nye he can "calculate the weight, the size, and the shape of the shadow of the mind [Nye is] standing in" may be referencing "My shadow weighs 42 pounds" from Al's song, "Fat".
Dyeing for Your Art: NicePeter had his barber take in his hairline so he could play Steve Jobs. Since his barber wasn't happy with doing it, he was generous, so NicePeter went back and made him do it worse.
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. PewDiePie cameos as the Russian Mikhail Barishnikov, though he is actually Swedish.
Fan Nickname: "Astrophysics Black Guy" for Neil deGrasse Tyson.
Fandom Nod: The basis for the rap battle, as it is the viewers who decide what comes next. At the end of most episodes, the YouTube comments that suggested the match-up are shown. There were at least three requests for Elvis Presley vs. Michael Jackson; one had over one hundred likes, and all three were acknowledged at the end of the respective video.
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.
While not quite said outright, ERB News Season 3.5 heavily implies Obama saw the rap battle between him and Romney.
Schedule Slip: ERB typically releases a battle once a month, but during the Summer 2012, there was a three month hiatus. Teddy Roosevelt calls them out on it and threatens to whoop some ass. To make up for lost time, new battles premiered every other Monday right up until the week of Christmas, and then picked up again in February.
Season 3 has fallen victim to this as well. The ERB news informs everyone that new battles won't air until May 5th.
So My Kids Can Watch: Unknowingly invoked - Chali 2na didn't quite get the impact left by Epic Rap Battles of History until he brought it up to his son, who didn't care that his dad wouldn't be able to go out with him for a movie!
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.
More like "Torch The Running Gag And Run", but The Stinger for the Rick Grimes vs Walter White BTS video has the "zombies" holding a mock funeral for the Heroin Joke, implying that we might never hear it again.
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.
According to the behind the scenes, Peter, who cameoed as Steve, wasn't planned to appear at first, which would have made this the first battle where he would not be in at all.
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.
The BTS gives two other lines that were considered for Al Capone's jab at Blackbeard's beard.
Peter: What beard joke do you like better: "From the likes of you, I've got nothing to fear/'cept maybe the evil beasties you've got nested in your beard." or "That rat's nest of a beard's trapped so many crumbs/you could feed your grubby crew lunch for a month."?
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.