Ice Climbers fusion glitch

The  fusion glitch is a glitch in Super Smash Bros. Brawl that occurs when the CPU Climber is in hitstun on the exact frame that the player successfully inputs Squall Hammer with the player-controlled Climber. The two Climbers will then be unable to be separated by moving around or being attacked. This glitch apparently changes how the game recognizes the Climbers' grounded and airborne states, which allows them to perform ground attacks in the air and enter animations such as the ledge grabbed animation where this would not normally be possible. The glitch also has the ability to teleport other characters around the stage.

Performing the glitch
A Squall Hammer must be initiated on the exact frame that Nana is in hitstun. There are a few ways to accomplish this:

As a general rule, when getting hit by an attack, if the Ice Climbers actually perform the Squall Hammer but get interrupted right away, then the input for the move happened too soon. If they don't perform the move, or it happens in full, the input happened too late.

When the primary Climber starts spinning around the secondary one in an odd fashion, the glitch is active. One way to verify for sure is to perform a Blizzard while standing still. If both Climbers point in the same direction, and behind them some scarce ice particles can be seen, the glitch is in effect.

Offline
One player chooses Ice Climbers while the other chooses Falco. Given the simplicity of the inputs, a single human can operate both controllers. Certain stages, like Final Destination, are ideal as there are few interruptions and a moderate amount of space.

From far away, around half to three quarters of Final Destination's length, Falco shoots the Ice Climbers with his Blaster while they are moving in the same direction as the shot. This is to make sure that it hits the secondary Ice Climber. When the laser is approaching, the game is paused. The laser should be about one Ice Climber-distance away from the secondary Climber or less. Until then, the game will need to quickly alternate between paused and unpaused. After the desired distance, the Climbers must have their directional input in the same direction, the special attack button is held, and the game resumes. If successful, the laser will hit exactly when the Squall Hammer starts, activating the glitch.

Online
One player chooses the Ice Climbers and another chooses Wolf. The preferred stage will contain unbreakable walls.

The Ice Climbers will walk into a wall continuously. Then, Wolf fires continuously at full speed; this rhythm is necessary to increase the chances of getting the activation in the right time. The Ice Climbers should then use Squall Hammer (while still holding against the wall) just when the laser is hitting. Because of input delay, it might be necessary for the players to experiment with the timing first, using the pointer above as reference (if the Squall Hammer is performed, Wolf should fire later, and if the Climbers are hurt, Wolf should fire sooner). Eventually, the glitch should be activated. Both players should check, otherwise another Squall Hammer will go off while the glitch is in effect, nullifying it.

Effects
During this state, the secondary Climber is fused to the primary one, following them wherever they go at all times. If Blizzard is performed while on the ground, both Climbers will face and attack in the same direction, although a small handful of particles will be able to hit behind them.

If the Climbers are KO'd, the glitch will continue taking place. This is an effective way to remove the secondary Climber out of their helpless state, should it happen. To cancel the glitch, the Climbers use Squall Hammer or Belay while the secondary Climber is not helpless.

If the CPU-controlled Climber uses an attack that puts him or her into a helpless state while he or she is in his or her airborne state, he or she will be permanently stuck in a helpless state even after touching the ground. If another character then footstool jumps off of the CPU climber while he or she is in the permanent helpless state, the character will be teleported to a different part of the stage.

Possible partial explanation
All special moves in Brawl draw their data from two files: the main moveset file of the character which handles action changes, physics values, hitboxes, and an auxiliary file that handles interactions such as the implementation of physics values and assembly-level coding.

While in freeze frames, no subactions can be executed. However, it is possible that code in the auxiliary files can be run. The glitch may involve the "Nana sticks to Popo to at the start of Squall Hammer" code from the auxiliary file executing, but no other code from the move can be run, resulting in a permanent stick.