It's a Feature
This important mechanic is in place in nearly every fighting game today due to an unintended side effect left in by the developers of the original Street Fighter II.
Combo/Cancel
If you could do this move back in the early fighting game days, you were already godlike. But now you can take shortcuts.
DP/z-motion
This SNK game started as a crossover game, but later turned into a mainline game for the majority of these characters.
The King Of Fighters
Hadouken! Smash players are confused about how many of these are included in their game.
Shotokan / Shoto
The mind games and mixups that occur around attacking and blocking when someone is rising from a knockdown. You hope to score this after a combo ender.
Okizeme / Oki
Occasionally, in the arcade version of Street Fighter II, when inputting this special move, It would be the wrong color. This later became intentional.
Red Fireball/Shakunetsu
Churning that butter. This motion may have single-handedly destroyed arcade cabinets back in the day, but for a huge pay off.
360/spd
This highly anticipated game flopped for it's poor state at release. There were many vocalized issues such as on-disc DLC and bad pacing with matches most often ending in time overs.
Street Fighter X Tekken
The "ultimate" fighting style, as made famous by the man in a Pink Gi.
Saikyo Style
POV: You just called an assist character out at the wrong time and now you're both in a touch of death combo. The commentators say this.
Happy Birthday
This Smash Bros. movement option is detrimental to success at tournament level and is performed by air dodging towards the ground at the right time.
Wavedashing
The name comes from the original motion necessary to perform this characters move. It was later simplified, but can still be used to input air specials close to the ground.
Tiger Knee
Pick your groove, cuz this is true love we're makin'.
Capcom VS SNK 2
Wind God Fists and Tsunami Kicks. This fighting style is seemingly tied to a bloodline.
Mishima Style Fighting Karate
Another word for excuses that smash players might make. If you ever go head to head with Reggie, please, none of these.
Johns
The ability to very quickly cancel a move into another move, before the first move completes its startup. Usually used for added range.
Kara
This "input" on the joystick is represented by a STAR symbol in the Tekken series.
Neutral / 5
The traditional violent finishers for this franchise had to be toned down in order to create a crossover with integrity for the invading cast.
Mortal Kombat vs. DC Universe
Guy, Zeku, and even Maki practice this style.
Bushinryu
The American Reset
A famous game-defining bug in Capcom vs. SNK 2 that allowed all special moves to be strike invincible.
This motion is named after a salty snack that would be fit for the king of fighters.
Pretzel Motion
Choose a character from Cave Story, Shovel Knight, or Binding Of Isaac in this indie inspired 2D fighting game.
Blade Strangers
This fighting style is not fictional and focuses on striking using fists, knees, elbows, and shins. No fireballs, though.
Muay Thai
An unblockable attack in Street Fighter Alpha 2, discovered by and named after American fighting game legend Alex Valle.
Valle CC