Yoshi's Island DS Glitches
Giant Egg
Go to Big Guy the Stilted's Castle, and get to where you need to be invincible Baby Mario. Become invincible and hit the middle ring while invincible, and then die. You will respawn with a huge Yoshi egg behind you which you can throw to make enemies turn into stars. I think this happens because the game checks how many eggs you have when you reach a checkpoint, and what kind, and sees the Yoshi egg that Yoshi is trapped in when you are invincible as a huge Yoshi egg gotten from eating a Fat Shy Guy.
Koopas go through walls
As shown in the picture, Koopas go quite a way into the wall when they get really close to it, and so does Baby DK when he dashes near a wall.
Crazy Number Ball
To cause this to happen, go to the world 3 extra level, which is called 'Number Ball Special or something'. Now, get one number ball, and go to the enemy pipe. Shoot it at the base of the pipe, and it will shoot underground, possibly (or not) coming back up the other side. If it don't come up, you will have to get the number ball all over again.
Egg in the wall
Go to the Fort of Moltz the Goonie, and reach the part with a gate and an egg plant the other side. Now lick the egg from the other side of the gate when it is open, and try and let the gate close quickly. The egg will float in the wall, like shown in the picture below:
Crazy Bouncing Eggs
In Hector the Reflector's Haunted House, go to where there is a arrow and a Shy Guy near a winged cloud as Baby DK or Peach. Now wait for the Shy Guy far above to go down the slope, and shoot an egg at the arrow to send it firing upwards. The egg, even though it should not bounce off walls, bounces off walls as if you used Baby Mario.
Floating Finish
In either Gilbert the Gooey's Castle, Hector the Reflectors Haunted House,Bowser's Castleor possibly another boss, fight the boss. Win, and make sure to run fast enough to slide off a platform. Instead of falling, you will just slide and stop in mid air, and you can watch Yoshi dance on nothingness.
Yoshi goes below the void
Fight Six Face Sal, and beat him. As you do, land on a Donut Lift platform. You will hear the ending music start, and keep falling off screen until you are taken out the level (from winning, not dying).
Space Bound Egg
In Big Guy The Stilted's Castle, get to where you must break a wall of spikes with eggs, but don't have full eggs and don't break any spikes. Wait until an egg is about to be sent through a spike by the egg plant, and lick at that time. The egg will shoot upwards to space like a rocket!
Go through a pipe
When you are against Priscilla the Peckish, make sure you end up on a green pipe when the battle finishes. You will then fall through the pipe, down the slope and to the bottom of the arena.
Congratulations! You.. lost?
In Yoshi's Island DS, I got to the final battle, and I died as Bowser did, and the congratulations music then played on the 'Try again' screen
Space Shooting Bats
In Six Face Sals Fort, go to the middle ring near the number ball slots. Go through it when a bat is in a wall, and the Bat will shoot upwards into the air, off the screen.
Non moving statue platform
On a platform that moves when standing on it, stand on the very far left or right. If done correctly, the platform just will not move.
Reach end early in 2-6
When you get to the part with some Amazee Dayzees, eat one and go upwards. Throw it onto the red M blocks, and bounce off it, and you can land by the goal ring and finish the level really quickly!
Maintain Baby Weight
Jump into the air while switching, and you will keep the weight of the last baby. This means you can make Baby DK float like Princess Peach with an umbrella and so on.
This found by Yoshuigi of Gamefaqs forums
Super Tap Tap Glitch
Go behind any Tap Tap in a level, and make sure it is off screen. Wait a decent time, and go after it. The blocks it would normally have destroyed should still be there, making the level easier.
Found by either dkrman or Yoshuigi of Gamefaqs forums.
Invisible Item in MouthHave an enemy in your mouth. Jump, and when you start flutter-jumping, spit it out. Look quick, and you'll see Yoshi still acting like something's in his mouth. To make it easier to see, keep licking it and spitting it out while flutter-jumping.
Eat an Egg?
I'm not really sure how to do this, but you just lick an egg close to it, and it'll act like you ate it. Well, it'll show Yoshi swallowing nothing. The egg won't go away or anything. I think you have to lick it while it's in mid-air to pull this off.
Invisible Ceiling
Run through 5-8 in the first room. While running, get to the 2nd arrow sign, and jump at a certain time while in it (in the foreground). If done right, you'll hit an invisible ceiling. Remember that you have to run at it to do this.
Bouncy Jump Glitch
Bounce into a bouncy in the ground. Make sure you slide across the ground (you can still hold left and right), and not fall off, or jump. When you stop, just jump. No matter how long or short you press the jump button, you'll jump a certain height. Note that you can actually jump any time during sliding, and still jump the same way. Just don't get off the platform in any way.