Ice Climbers fusion glitch
The Ice Climbers 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.
How to perform
A Squall Hammer must be initiated on the exact frame that Nana is in hitstun. There are a few ways to accomplish this, and the following are suggestions.
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
Have one player be the Ice Climbers and the other Falco. Given the simplicity of the inputs, a single human can operate both controllers. Pick a stage with few interruptions and a moderate amount of space, like Final Destination.
From far away, around half to three quarters of Final Destination's length, have the Falco player shoot 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, pause the game. You will want the laser to be about one Ice Climber-distance away from the secondary Climber or less. Until you get there, quickly unpause and re-pause the game. After the desired distance, keep holding in the direction the Climbers are moving, hold the special move button, and unpause the game. If successful, the laser should hit exactly when the Squall Hammer starts, activating the glitch.
Online
Have one player be the Ice Climbers and another be Wolf. Choose a stage with walls, preferably avoiding Shadow Moses Island, given that that stage's walls are destructible.
Have the Ice Climbers player walk into a wall continuously. Then, have the Wolf player fire continuously at full speed; this rhythm is necessary to increase the chances of getting the activation in the right time. The Ice Climbers player should then press the special move button (while still holding against the wall) just when the laser is hitting. Because of input delay, it might be necessary to experiment with the timing first, using the pointer above as reference (if the Squall Hammer is performed, activate later, and if the Climbers are hurt, activate sooner). Eventually, the glitch should be activated. Take care to check, or else you might perform another Squall Hammer 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, 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.