Stun Jacket is a glitch that in which stored hitlag, resulting from a missed counterattack, is applied to the next attack. This glitch was first introduced in Super Smash Bros. Brawl, and has since returned in the subsequent games.
When a move with a disjointed hitbox is countered, but the counterattack doesn't connect, the 16 frames of hitlag are "stored". This is due to the game registering that the opponent has yet to receive the hitlag due to the counterattack not landing, and will apply them in the next attack. However, the hitlag is non-additive; any attack that normally has less than 16 frames will apply said amount. For moves that have more than 16 frames, the hitlag is unchanged. If shielded, Stun Jacket will apply 16 frames of shieldstun on the opponent. Interestingly, Stun Jacket is also consumed by grabs and pummels.
All characters that possess a counter (except for Bayonetta, Corrin, Little Mac, Lucario, and Shulk) can make use of Stun Jacket, provided that the miss the counterattack from a move with a disjointed hitbox, barring attacks that involve swords, and projectiles.
Due to Stun Jacket's properties, it is possible for moves that usually have no combo potential to be able to lead into potential true combos.
Stun Jacket was originally in Ultimate, but was removed in update 3.0.0.
Examples of moves that trigger Stun Jacket