Have you discovered any bugs, glitches, or tricks that you would like to share, but you can't post it in the SMW data thread because it said not GLITCH or BUG? Well, here's the place! (You could use these glitches in your hack)
Jump off a P-Switch
By jumping at the exact frame Mario lands on a P-Switch, it can be used as a platform to jump off. Note that the Y button cannot be held on this frame - otherwise, Mario will just pick up the P-Switch again. Because of that, either the P-meter must be recharged after landing, or the jump should be followed by the hopping glitch.
Land on a midair P-switch
When dropping a P-switch, it will travel with the same horizontal speed that Mario does. To land on it, Mario must slow down a bit before releasing it, and then, regain the lost speed to catch up. The best way to do this is by dropping the P-switch at a low point of Mario's oscillating sprinting speed, and then using the high point of the oscillation to catch up to it in mid-air.
Note: When dropping the P-Switch, it must be dropped, not kicked. This is done by not holding → when releasing Y.
Springboard clipping abuse
When Mario jumps on a springboard to compress it, the game ignores most kill triggers, including walls. In addition to this, Mario will remain "stuck" to the spring while it is compressing. With these properties, Mario can go through solid objects by throwing a springboard in the air, and jumping on it as it falls past a corner.
Springboard jumping abuse with Yoshi
If Yoshi eats a springboard on the same frame he steps on it, he will soar upwards very fast, ignoring most boundaries. The speed and duration of of takeoff can be tweaked with the ↑ and B buttons.
The key is a unique object - it can be stood on, and it cannot be destroyed. Because of these two properties, it can be jumped off of, and also carried with in the same jump. When near a wall, this process can be repeated:
Jump with the key.
Drop the key near the peak of the jump (but not at the very peak).
Land on the key in mid-air (without holding Y).
Jump off of the key, and start holding Y on the same frame.
You will now have jumped in mid-air and be holding the key. Repeat.
By this process, walls can be scaled and jumping can pretty much go anywhere, as long as a key is handy. Note that if the key has too much downward momentum, it can be jumped off of, but not carried at the same time.
Using a key to enter mid-air pipes
The key can be used as a temporary platform to fool the game into thinking that there is ground near the pipe, and enter it in mid-air. It's also possible to bring the key with you through the pipe by holding Y on the frame before heading into the pipe.
Using a key to pass through invisible coin blocks
By standing on the key as it bounces, the height can lift Mario up far enough to jump above invisible coin blocks that would otherwise appear when jumped from the ground.
Hit a question block while moving downwards
If Mario passes though the corner of a question block enough, the game will think he is far enough inside of it that it gets hit and releases its contents.
Duplicating question blocks
By throwing an enemy or object at a question block very close to it, a question block can be duplicated near it, unveiling the item, leaving the original block untouched. This can be done to either side of the block, above it, or above-diagonally to either side.
New items from old blocks
The game differentiates certain items by the block's horizontal position in the level. For example, blue P-switches and gray P-switches are actually the same object in the game, but determined by an odd/even horizontal position of the question block which contains it. Thus, by duplicating a block to the side with a p-switch inside, it will give a P-switch of the other color.
An interesting item-group abused by this trick is the group which contains keys, p-balloons, and yoshi wings. See Super Demo World's Crystal Star World level in the short run for an example of this.
Yoshi Wings ↔ P-Balloon ↔ Shell ↔ Key ↔ Yoshi Wings