Where am I?
In World 3-1 or 4-7 (easiest in 4-7), find the Spring Ball that leads to a bonus area. Spring off of the ball, but hold Left on the Control Pad as you're flying upwards. Instead of going to the bonus area, you might end up high in the sky of another area. If you are carrying any Huffin Puffins from 4-7, they'll turn into glitched red blobs of some sort. Anyway, once you make it here, repeatedly flutter jump to the right until you can't go any farther, then flutter to the left as far as you can. Keep going back and forth, and eventually you'll scroll up some land. You have now arrived back in World 1-1! You'll still have everything you collected from the level you came from, and anything else you get will be counted towards the level you came from. Although you can rack up MORE than 100 points from playing in two levels, the game will not award you more than 100 when you pass through the goal ring.
-- [Sent in by Nicholas Harvey]

Compressed blocks
In World 2-1, get past the part where you're automatically scrolling up a tall shaft with blocks constantly falling down. Go to the right until you see a big Spring Ball, and a tall wall above you to the right. Shoot an egg up the right side of the wall to reveal a Winged Cloud. Shoot the cloud for a ! Switch. Step on the Switch, then press Down on the Control Pad where you see a red arrow pointing down. Once you enter the bonus area, just go up the pipe on the left to exit. Now, go back to where the tall shaft was with the falling blocks. The falling blocks have reset, and you'll see the big smashed mess of blocks fall all at once, making a long earthquake as they hit the ground. If you go down the shaft to explore, you won't be able to get back up...
-- [Sent in by Nicholas Harvey]

Message Block glitch
In 2-7 (Lakitu's Wall), eat at least one fat Shy Guy to give Yoshi a huge egg. Go through the course up to the part with the Message Block and the car morph bubble thing. Shoot the big egg straight up at the Message Block. The egg will hit the block twice, causing the screen to go crazy. Yoshi will disappear, the stars will come to the foreground, and you might even get a small message text box lingering on the screen. Even though Yoshi is gone, you can still move around. For a real challenge, try finishing the level! If you make it through the goal ring, the game will freeze up.
-- [Sent in by Chomp]

Koopa in a Hill
In World 4-5, go to the last big hill and kill the two Beach Koopas (Koopas without shells). If the Koopa with a shell is walking up the hill, jump on him so when he pops out of his shell, he goes up the hill. Let him climb back in his shell, and while he is still shaking stomp him again. If he goes upward, he will be stuck in the hill!
-- [Sent in by Dreemz]

Yoshi Copperfield
In Extra 3, there is a spinning platform next to the first Flower in the stage (which is blocked by a wall). Jump on the spinning platforms, and face left. When Yoshi's back hits the wall, rapidly tap Left on the Control Pad a few times, and Yoshi will go through the wall!