Pick up nothing
After you throw an enemy into another, jump on the cluster of defeated foes and quickly press B as if you wanted to pick them up. Instead of picking them up, your character will pick up nothing! The game acts as though you are really holding something though, which means you can't go in doors unless you press B to throw your "nothing" away. (To make it easier on yourself, try hitting the second enemy on a bounce. This will give you more time to jump onto the dying foes.)
-- [Sent in by joseph.kendall@home.com]

Suicide door
While entering a door, pause the game before the screen changes. Enter the suicide code (Up + B + A on Controller II), then unpause the game. Instead of dying once, you'll watch your character fall through the floor and die for each extra life remaining. Game over!

The power of Pokey  [screen shot]
Lead Pokey off a ledge, and he'll keep walking in a straight line, but in mid-air. On a related note, Pokey will also walk through walls if you can lead him to one. When he walks through the wall in World 2-2, his head pokes through the top; pick it up and the rest of his body will disappear.
-- [Contributing: nintendoexpert89]

Door of doom
In 5-1, pull out the Potion, and use the rolling logs to get on top of the tall wall before the door to Birdo. Throw down the Potion so the door forms off-screen, then take a few steps and press Up to go in the door. You will die as soon as you enter Sub-Space.
-- [Sent in by TeamRocket2002]

Life meter glitch  [screen shot]
When you pull up a Rocket, pause the game before the Rocket takes you to another screen. Perform the suicide code (Up+B+A on Controller 2 while paused, then unpause). When you unpause, your character will have no dots filled in the life meter, but the Rocket will take you up to the next screen, explode, and drop you off as usual. But now, you'll be "small" with no dots in the life meter, and the music will turn off (but not in 4-1, because the music changes). Grab a Heart, and it will fill in two dots, but you'll still be small! Grab another to fill in the third dot, and to become "big."
-- [Found by David Wonn]

Trapped  [screen shot]
When fighting Fryguy, break him down into four "baby" Fryguys. Now try to kill one as you get hit by another at the same time. After you kill off the rest of them, the door won't appear.
-- [Sent in by sin@iconnect.net]

Illusion items  [screen shot]
Throw down a Potion on top of some grass. Enter Sub-Space, and press Up to exit. As soon as the door opens, quickly and repeatedly press B. If your timing was right, the item that you pulled out will be on top of your head. You can't throw it, but if you go in a door, or up a vine to a new area, the item will become real and will be sitting in your hands. If you get killed while carrying the illusion item, you'll have a real item in your hands when you retry the stage!
-- [Found by Jeff Walker]

A chip off the old Rocket  [screen shot]
Do the "illusion item" trick above, and then go and pull up a Rocket. When it flies upward, your item will stay where it was, and the upper right corner of the Rocket will be missing.
-- [Sent in by sin@iconnect.net]

POW madness  [Screen shots: 1, 2, 3]

• In World 7-1, pick up the POW and go to the end. Jump on an Albatoss and ride it all the way to the left. Throw the POW, then immediately pull up the Rocket to make it shoot up super-fast.
• Get a POW block and find some logs rolling down a waterfall (try World 1-1). Throw the POW Block, and if you are standing on one of the logs when the POW Block hits the ground, the log will go straight up and off the screen.
• You'll get similar results as the glitch above if the POW block hits the ground while you're standing on a Crystal Ball (try World 1-3), Mushroom (3-3 Sub-Space), or Turtle Shell (1-1).

• Throw a POW block, then quickly pull up some vegetable and throw it. The veggie should fly high in the air.
• Throw a POW block, then quickly pull up a Bob-Omb (try World 5-2). He should drop right through the ground. If you go in a door, die, or otherwise leave the area, you'll experience a delayed Bob-Omb explosion...

Warp backwards
In World 6-1, get a Potion from the first vase. Take it to the grass between two cacti that yields a Potion. Do the "Illusion item" glitch as explained above, and make sure you get that Potion as the illusion item! Now go into the cave with all those vases. At this point, your illusion Potion should've turned real by entering the cave. Throw it down, and while in Sub-Space, go down one of the vases. You'll warp to 6-1, the same level you were in! (Any vase that cannot normally be entered in Sub-Space turns into a backwards Warp Zone with this glitch.)
-- [Found by David Wonn]

Disappearing Mushroom Blocks
This trick is good to do in World 2-3 after beating Tryclyde, where you'll have a doorway and lots of Mushroom Blocks to work with. Put one or two Mushroom Blocks horizontally on top of the door. Pick up another Mushroom Block, and as you're moving, try to throw the Mushroom Block into the bottom corner of one of the Mushroom Blocks sitting on the door. If you throw it in the right place, it should go in the exact same spot as one of the Mushroom Blocks sitting on the door. The one you threw replaces the one that you threw it into, I guess. This glitch is sorta hard to explain... but once you pull this off, it's easy to duplicate.
-- [Found by Deezer]

Flashing while climbing a vine
On vines that go all the way to the ground, get on or in front of it, and press and hold Down until you start to flash like you're ready for a super jump. Start climbing the vine, and you'll still be flashing as you climb! If you press Left or Right while on the ground you will cease flashing.

Sub-Space music in the overworld
Get Starman and immediately enter Sub-Space. Stay there until your time is up and you're thrown back to the overworld. If you're still invincible when you come out, the Sub-Space music will keep playing over and over.

Enemy elevator  [screen shot]
If you throw an enemy, then hit it with another enemy while it's still upside-down, both enemies will shoot straight up. One way to do it is to find a wall or pillar. Hold an enemy while standing on top of another enemy. Press B twice quickly, and if you bounce the first one off the wall you'll have a good chance of pulling off this bug.
Another way is to throw a Shyguy into quicksand, then quickly throw another Shyguy onto the one drowning in the quicksand (try World 2-1). I think they stay upside-down longer while drowning in the quicksand, so this might be an easier way to do it.
-- [Found by David Wonn]

Bob-ombs from Mushroom Blocks  [screen shot]
Find one of those vases covered with a Mushroom Block, that when you take off the Mushroom Block, Bob-ombs come out. Pick up the Mushroom Block and throw it down to the side of the vase leaving a one-block-width gap between the Mushroom Block and the vase. Stand behind the Mushroom Block so the Bob-ombs get stuck between the vase and the Mushroom Block, and eventually they'll blow up the vase. Now take the Mushroom Block and throw it where the vase used to be. Scroll the Mushroom Block off the screen, then scroll back. Bob-ombs will emerge from the Mushroom Block!

Put holes in Chains / Change the color of Mushroom Blocks  [Screen shots: 1, 2]
Pick up a Mushroom Block and jump towards a chain. When the chain is directly behind you, throw the Mushroom Block. It should get stuck in the chain and change to a greyish-blue color. If you jump onto the Mushroom Block and pick it up, the Mushroom Block will revert back to its original color, leaving a hole in the chain in the process! Also, once you get on top of the Mushroom Block, you can do the "flashing while climbing" bug on the chain. This also works on vines and ladders.

Floating Mushroom Blocks

• In doorways created after you beat a boss in any x-3 level, you can throw Mushroom Blocks on top of sunlight emanating through the door.

• Get at least 2 Mushroom Blocks, and put one in a corner against a wall. Pick up the other one, run towards the wall and jump, and make sure you are still moving towards the wall in mid-air. Right before you touch the wall, throw the Mushroom Block and then try to land on top of the one in the corner. If you did this right, the Mushroom Block you threw should be behind you. Now press B to pick up the Mushroom Block you're standing on, and then jump. The Mushroom Block you threw is now floating in the air! You can do this over and over again until the floating Mushroom Block is too high to jump onto.