The ceiling glitch is a glitch in Super Smash Bros. Melee that causes a character knocked upward to get hit by an invisible "ceiling", causing them to be "stage spiked" downward.
Explanation
The ceiling glitch occurs when a character is hit while they have Attacker Shield Knockback Velocity. When a character hits a shield with a grounded move, a force is applied to their character that pushes them back. The velocity from this knockback quickly dissipates due to traction. Usually, it dissipates so fast that the shielding player does not have enough time to counter attack while the attacker has Shield Knockback Velocity. Powershielding, edge canceling, shield smash DI, and other techniques can be used to allow one to counter attack in time.
What happens to the victim is that their vertical velocity is instantly changed to zero within the first few frames of hitstun. This is what makes it look like a collision with a ceiling, as seen in the teams gif below. However, if the move used sends at a lower angle, the trajectory can look like a semi-spike, as seen in the Fox/Falco gif and Pikachu/Falcon gif below.
The ceiling glitch can happen to characters off stage if they leave the stage while they have Attacker Shield Knockback Velocity. Normally, move animations prevent characters from passing ledges, which makes this rare. Most special moves will, however, slip over ledges. This can be observed in the Brinstar Acid Glitch video.
Marth's and Roy's counters can also cause the ceiling glitch somehow.
This glitch resulted in S2J losing a set to Abate at The Big House 5 on October 4, 2015. [1]