Its impossible to use Chaos as a playable character, however its possible to play as him if you input some cheat codes.

The bedroom seen in the intro and in the scene shown after completing 1P Game has a door that is always completely unseen. Since the camera focuses mostly on the table where Master Hand puts the dolls on, the door stays hidden behind the camera.

There's a build date that can be found in the ROM at 0x4C61. It can be displayed in the "Options Menu" by selecting sounds 0, 5, 0, 3, 6, 0 in the "Sound Test".

Eve is likely based on the story of Adam and Eve, the original humans who ate the forbidden fruit, often considered to be an apple, which cursed humanity with knowledge of sin. Her name, the bite mark where her right eye was, and that her head is an apple possibly reference it. It could also be a reference to the expression "an apple a day keeps the doctor away", since she's a nurse and has an apple for a head.

Originally, the in-game three-day system was going to be a week, but Eiji Aonuma, the game's director, thought that player's would be annoyed having to go through an entire week again and it would be hard to remember what happens each day.

Inside the bedroom in Mama's House at Romani Ranch, a toy version of Ganondorf's horse from The Legend of Zelda: Ocarina of Time can be found discarded in the corner near a cupboard.

A robotic Operating Buddy (R.O.B for short) can be found in the backroom of the Curiosity Shop. This can be seen when the player uses the spyhole.

In the bomb shop in West Clock Town, a large apparatus resembling the front of a Nintendo Gamecube can be seen, complete with controller and memory card ports. To the right of it, the player can find an Ultra Hand, a toy released by Nintendo in the 1960's.

An NPC character that appears early in the game is known as Cleric Joy, which is a nod to Pokemon's Nurse Joy. Clerics are depicted as healers, which fits Nurse Joy excellently.

During the final boss, there are flower symbols on the floor. The player can actually use these as Deku launch pads if they equip the Deku Mask.

Some of the Boxing Ring Stage titles contain references and easter eggs.

Luigi's, "The Eternal Understudy", is a reference to his unlock screen in the original Super Smash Bros..

Fox's title in the European version "Never Gives Up! Trusts His Instincts!" is a reference to a line in Starfox 64 said by Peppy in the Asteroid Belt level (and later on said by Fox's father).

Kirby's European title "Gritty in Pink" is a reference to the movie "Pretty in Pink".

Sonic's European title "Speed is his Game" is a reference to one his popular quotes "Sonic's the Name, Speed's My Game!".

Pikachu and Jigglypuff's European titles are actually just their Pokedex names.

In the demo versions, the song "Escape from the City" was an instrumental. Whether the song was originally planned to have lyrics or not is unknown. This instrumental is slightly different than final version's instrumental. Part of the demo version was used in the "Classic City Escape" remix in Sonic Generations.

The "Hi Skitty Doll" in Sea Mauville is a reference to Hello Kitty.

The couple in Sea Mauville that give you the Destiny Knot are named after the parents in Malcolm in the Middle: Hal and Lois.

When the NES was first released in North America in 1985, it was only possible to purchase it from a few stores in New York and Los Angeles. This came soon after the great video game crash of 1983.

People expressing interest in the console wrote to Nintendo of America who provided them with information packs about the console, the games and peripherals that were available, and the then only three stores in North America which offered mail orders for the console.

"Fortress" from the Metal Gear Solid: Portable Ops soundtrack and "Road Rage" from The Bourne Supremacy soundtrack sound very similar.
Metal Gear Solid: Portable Ops was made in 2007, after The Bourne Supremacy which came out in 2004.

As an homage to the Vietnam War film, Apocalypse Now, whenever the player chooses to send out a helicopter, Richard Wagner's "Ride of the Valkyries" plays in the background.

The famous Baroque Era composer Johann Sebastian Bach is listed as one of the game's composers, and the in-game sound test attributes his talents to several specific music tracks.

In the Scarescraper, one of the bosses is called Fright Knight, his description reads;

"Rumor has it that this Greenie comes from the middle ages, where he was a knight searching for the kidnapped princess. Sadly, wherever he went, she always seemed to be in another castle"

The last bit is obviously a nod to Super Mario Bros. on the NES, where Princess Peach was always in another castle.

By ignoring the first dragon encounter after leaving Helgen, it is actually possible to play through a vast portion of Skyrim without bringing the dragons back to life. This can be particularly useful, as random dragon encounters can kill quest-giving NPCs and otherwise disrupt missions the player might be undertaking.