Stun Jacket is a glitch 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 reappeared in Super Smash Bros. 4 and briefly appeared in Super Smash Bros. Ultimate before being removed in version 3.0.0.
A Stun Jacket can be performed by characters with a Counterattack, with the exception of Bayonetta, Corrin, Little Mac, Lucario, and Shulk. When a move is successfully countered, but the counterattack does not connect with the opponent, the roughly 16 frames of hitlag that would have activated if the counter hit is stored or "worn" on the opponent. The next time the opponent is hit, they will receive those 16 frames of hitlag, making them vulnerable to follow up attacks. 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. The hitlag is non-additive, which means that any attack that normally has less than 16 frames will now apply said amount. However, for moves that have more than 16 frames, the hitlag is unchanged. If the opponent is shielding while Stun Jacket is activated 16 frames of shieldstun will be applied instead. Interestingly, Stun Jacket also applies to grabs and pummels, but doing so is not recommended as the hitlag will end before the fighter can follow up with anything substantial. 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.
Examples of moves that trigger Stun Jacket