Bad Ending: Beat the game with at least one continue.
Guaranteed Victory vs CPU
If you have a controller with a "turbo" option for buttons A or B, you can use the following exploit against the CPU so that you always win!
First, when you turn on the game and Yoshi appears in front of a blue screen and says "Nintendo," press B, A, L, L. You should hear a confirmation code when you input the cheat correctly. This will remove the "freeze" that occurs when you or your opponent makes a combo or a chain. In other words, the blocks never stop rising. After entering the cheat, activate turbo for either A or B. Once you are competing against your CPU opponent in splitscreen, place your cursor over any two blocks or one block and an empty panel, and hold down turbo. The panels will switch back and forth so quickly that your blocks will not rise, nor will garbage blocks fall on you! Eventually, the speed of the rising blocks will surpass the CPU's ability to get rid of them and you will win!
Although it takes a while in later stages, you can use this exploit to beat every difficulty in VS Mode.
How to control the cursor in "How to Improve Mode"
In any of the sessions hold down the "R" button. You can now control the cursor and try out the chains, combos, and skill chains on your own.
Move Yoshi on Credits Screen
After you've beaten the game on Medium difficulty or higher and the credits are rolling, hold SELECT and you can use the Control Pad to move Yoshi around in the background.
Time Trial x? chain bug
In Time Trial, you get progressively more and more points for larger chains until the 13x chain. However, if you keep a chain going any longer than the 13th link (x?), you don't get any chain-link bonus points. For this reason, you should never let a chain go past the 13th link in time trial.