Cardboard Cutouts:
All of the scenery in the Yoshi's Story arena is designed to look like it's made of cardboard. In fact, the actual stage itself is set inside a cardboard box. If you jump as high as you can and zoom in on your character, you can see the top corners of the giant cardboard box. It's quite a small world after all.

Submitted by Arvid "Olimar" Keemink

Daisy's Third Eye:
A lot of people claim that they have eyes on the back of their heads. But not very many people can actually back that up... In the trophy gallery, choose the Daisy trophy. If you rotate the camera and move it to the back of her head, you can zoom in and see a third eye behind her right ear. Creepy...

Rainbow Falls:
Most players have probably already noticed this, but I've decided to throw it in here anyway... In the Kongo Jungle arena, the mist from the waterfall produces a small rainbow, usually visible at the bottom of your screen. Just another one of those details thrown into the game for completeness.

Klaptrap Country:
Veterans of Donkey Kong Country for the SNES surely remember these little bastards, nipping at your heels wherever you go. They've made a comeback in Melee, although they're only visible for a few seconds at a time. You can spot the blue species swimming in the water below the Jungle Japes stage, while the red variety can sometimes be seen flowing down the river, and over the falls in the Kongo Jungle.

Dangerous Battle:
If you walk too far off the screen in the Mushroom Kingdom arena, you'll disappear and die. There are actually large warnings posted on both ends of the arena. If you pause the game and angle the camera, you'll see large DANGER signs.

Meddling with Mario:
There's a hidden detail in the Metal Mario trophy that you would normally overlook. If you've earned the Metal Mario trophy, head to your Trophy Gallery. If you zoom in, and check out the trophy from the right angle, you can actually see a reflection of the Yoshi's Island stage. No, it's not someone's overactive imagination. I've included a shot from Yoshi's Island to prove it.

Submitted by NSALackey

A Hole in One:
Nothing's too good for a princess... That's why Princess Peach has her very own golf course on the vast lawn that surrounds her castle. You can spot the green from the Mushroom Kingdom in Adventure mode, off in the background. A little bit of a flashback from Mario Golf. I wonder if there's also a go-cart track nearby...

Pepper Poster:
While you're fighting on the Great Fox as it flies over Corneria, you can watch as you pass over the buildings in the city below. One some of the buildings, you'll spot a banner with the face of General Pepper from Star Fox.

Look Out Below!
To the far left of the Jungle Japes stage, there's a barrel outhouse on the platform. If you pause the game and angle the camera from below, you'll see a drop hole in the bottom of this primate potty shack. Just be careful not to fall down into the water there now...

Capsule Label:
Here's a detail that I just happened to find by chance, located on the item capsules themselves. If you pause the game and zoom in all of the way, you can actually see the word "CAPSULE" inscribed.

Cranky's Cabin:
Many players have probably noticed this already, but for those who haven't, you can see Cranky Kong in the Jungle Japes arena. The silhouette of Donkey Kong's father can be seen inside the hut on the center platform. Cranky rocks in his rocking chair, and occasionally stands up to pace around the room.

Shooting Sight:
This is a detail that you don't normally see. Whenever you're firing the Super Scope weapon, pause and zoom in for a closer look. Some characters will actually have one of their eyes closed while shooting. Talk about detail.