Pokémon Diamond & Pearl
Pokémon Diamond & Pearl
Pokemon's Creation & Game Freak - Did You Know Gaming? Feat. Tamashii Hiroka
Pokemon & Religion - Did You Know Gaming? Feat. Jimmy Whetzel
Pokemon & Science - Did You Know Gaming? Feat. JonTron
Pokemon Part 3 - Did You Know Gaming? Feat. Egoraptor
An item called the Azure Flute is present in the game's data, but can't be legitimately accessed. Hacking the flute into one's inventory triggers a battle with a level 80 Arceus in the Hall of Origin, an otherwise unused area hidden above Spear Pillar. Both the Hall of Origin and the battle with Arceus feature unique themes, which also go unused in the normal course of play.

The Azure Flute was intended as an event item to be given out to players, much like the Event Card and Oak's Letter (which allow the player to fight Darkrai and Shaymin, respectively). However, Game Freak scrapped the idea upon concluding that it would be too confusing for players. Instead, Arceus was directly given out at events.

Arceus' battle theme would later be used for the Sinjoh Ruins event in Pokémon Heart Gold & Soul Silver where players can obtain a Dialga, Palkia, or Giratina if they have an Arceus as the first Pokémon in their party. This event additionally contains code allowing it to be triggered by an Arceus from the Hall of Origin, indicating that Game Freak still considered distributing the Azure Flute this late into Generation IV. Spoiler:The Azure Flute item and its corresponding event would eventually be implemented in Pokémon Brilliant Diamond & Shining Pearl, where players can access both by importing save data from Pokémon Legends: Arceus after catching Arceus there.
Contributed by game4brains
There are certain days coded into the game's files that alter Pokemon spawn rates.

On August 6th, August 9th and September 11th, Pokemon are 10% less likely to spawn in the wild. These dates coincide with the atomic bombings on Japan, and the September 11th terrorist attacks on the United States.

On certain major holidays such as Christmas, New Years and Culture Day (November 3rd in Japan), Pokemon are 5% less likely to spawn in the wild possibly to discourage gameplay during holidays.

33 other holiday dates were found to have a 5% increase in wild Pokemon spawns, such as St. Patrick's Day (March 17th), Independence Day (July 4th in the United States), Bank Holiday (August 28th in the UK) and Christmas Eve (December 24th).
Contributed by KidDivinegon
Lopunny is based on the Playboy Bunny caricature or similar "bunny girls" seen in popular culture. The bunny girl style is a popular appearance in much of Japanese adult media.
Contributed by PowerRing
Pokémon Diamond and Pearl contain many references to Internet memes and chatspeak. This is possibly because the lead translator, Nob Ogasawara, is a member of the Something Awful Forums.
Contributed by xaxdxaxmx
On the back cover of the Australian version of Pokémon Diamond, it states:

"...as you search for the Pokémon that rules space in Pokémon Diamond Version."

This is an error, as the legendary Pokémon of Pokémon Diamond is Dialga, who has the power to control time. This mistake is not present on the box art of other regions.
Contributed by xaxdxaxmx
In the Canalave Library, the story "Sinnoh Folk Story 3"³ differs between the Western and Japanese releases of the game. The Western version tells about how Pokémon and humans were once very close and treated each other as equals. The Japanese version however tells about how Pokémon and humans used to marry each other which was considered normal because they once appeared similar.
Contributed by KnowledgeBase
When the total play time counter reaches 999:00, it skips over to 999:59 as a result of a glitch, thereby skipping nearly an entire hour in the counter.
Contributed by game4brains
On the Meister's Island, if the player trades away Finneon the player would be stranded if the player does not have or is unable to obtain a Pokémon that can transport the player.
Contributed by Berry
When fighting Barry on route 203, he will say "Don't get cocky, kid! This isn't over yet!", a likely reference to Star Wars' Han Solo, who says something similar to Luke Skywalker.
Contributed by nolanms1
Rotom has more forms than any other non-legendary Pokemon, and when you battle it in the generation IV games it's the only non-legendary Pokemon that uses a legendary Pokemon encounter theme.
Contributed by Funland47
In the non-English European versions of Pokémon Diamond and Pearl, Registeel's sprite was changed because the position of its left arm was similar to the Nazi salute. In Pokémon Platinum, the sprite was changed for all regions to the continental European variation. This change was retained in HeartGold and SoulSilver.
Contributed by CodenameToast
The title screen music for Pokemon Ruby, Saphire, and Emerald can be found in the code for Diamond and Pearl. It is labeled "MIDI HEX 0003" and was likely used for testing purposes during development.
Contributed by WhiteSamurott
There are 9 unobtainable items within Pokemon Diamond & Pearl. Some were removed entirely, some were replaced with updated versions, and some are only available under certain conditions.

These items and descriptions are as follows:
Park Ball - Captures Pal Park-transfered Pokémon without fail (obtainable in game but only in Pal Park)
Cherish Ball - Normal chance of catching a Pokémon (Pokemon received from official distributions come in this ball)
Boat Ticket - Ticket for the Luxury Liner SS Anne
Contest Pass - Allows entry into Pokémon Contests
Magma Rock - Mystical item that maintains the Stark Mountain's volcanic activity
Red Chain - A mythical item created by the Sinnoh Legendary Trio to awaken Dialga/Palkia
Rule Book - User's guide to online trading
Seal Sack - Holds up to 10 Seals
Treasure Sack - Holds treasures found Underground
Contributed by AussieYak
Within Route 218, inside one of the hallway type buildings, there is a guard who will tell you how thirsty he is. However, bringing him a drink of any kind won't do anything.

This is a reference to the original games, Red/Blue/Green/Yellow, in which the player must bring the guards a drink to enter Saffron City.
Contributed by PheonixGRX
On January 12th (Director Junichi Masuda of GameFreak's birthday), Fly up to Snowpoint City and instead of the usual flurry snow storm, there will be glittery diamond snow. There are also a number of other dates that this Diamond Dust can be seen.
Contributed by PheonixGRX