If the player character is male and goes to the Rondez-View Ferris Wheel in Nimbasa City in an autumn month, they may battle an NPC named "Nursery Aide Enatsu" (June in English). In the Japanese versions of the game, June is actually a man disguised as a woman in order to secure a job as a Nursery Aide.
Cheren is the first rival to become a Gym Leader within a generation. It took the passing of a full generation for Blue to become a Gym Leader.
In the files for the Pokemon World Tournament, there are two complete text strings for Hilbert and Hilda, the player characters of Black and White. This suggests that they were originally going to be present in an unidentified tournament, and they were also going to speak in full sentences, unlike Red who just speaks in ellipses. Their names are set as Hilbert and Hilda, unlike the Memory Link referencing your past character.
Similar to Pokemon Diamond, Pearl, Platinum, HeartGold, and SoulSilver, Diamond Dust appears in-game. It appears in Iccirus City and Route 8 on various dates, including December 31st. Seeing the dust earns you a medal on your next visit to the Medal Master.
Despite Route 10 being removed from the game, its unique track can still be heard through legitimate means. Visiting Icirrus City during Winter and speaking to the NPC who claims to be the sound designer (located in the ex-Rocket Grunt's house) will change the background music to Route 10's theme.
There's a female trainer named "Guitarist Billy Jo," whose name and appearance resembles a female version of Billie Joe Armstrong, guitarist and singer of the band Green Day.
There's a Lady in Humilau City that says "This dress is comfy and easy to wear," referencing a line from the original Pokemon games where Youngster Ben says "Hi! I like shorts! They're comfy and easy to wear."
Remnants of Route 10, removed from the map in Black 2 & White 2, are still visible in the game's code. The textures, models, and collisions remain, but the scripts, text, and wild Pokemon data are not present.
In Pokestar Studios, there is a specific place where you can surf on the base of a tree. While this is just a glitch and serves no use, it is reminiscent of a multitude of similar surfing glitches in Pokemon Red & Blue.