Sudden Death occurs when any VS. mode match ends in a tie. A tie happens when at least two characters or teams have the same final score at the end of a Time match, the same amount of stocks at the end of a timed Stock match, the same amount of coins at the end of a Coin Battle, or the same total score at the end of a Bonus match. In an untimed Stock match, Sudden Death is possible if all remaining characters lose their last stock on the same frame.
When Sudden Death occurs, the words "Sudden Death" will appear onscreen with the announcer calling it and the match will restart with only the players involved in the tie, all of whom are given one stock and 300% damage, making them very easy to KO in a single hit. In effect, this makes it so the first player to land a hit on their opponent(s) wins Sudden Death and therefore the match. With the exception of the original Super Smash Bros., items will not spawn at this time even if they are turned on.
If nobody wins after roughly 20 seconds, Bob-ombs begin to fall from the sky to speed up the match in all games besides Smash 64. Unlike the Bob-omb items, these immediately explode on contact with any surface or player. As all players have 300% damage, getting hit by a Bob-omb is essentially an instant KO; however, with precise timing, players can grab Bob-ombs in midair with air dodges. Prior to Smash 4, it is possible for a falling Bob-omb to instantly KO a player off a side blast line while the other player is currently being sent flying by a Screen or Star KO, causing the latter player to win the match in the middle of their KO animation; in Smash 4, Star and Screen KOs no longer occur during Sudden Death.
Should a tie occur within Sudden Death itself, which can only happen if all remaining characters are KO'd on the same frame, the match will end and one player will be declared the winner. In the first two Super Smash Bros. games, placement is based on port priority: in the original Super Smash Bros., the winner is the player with the highest port number (whose controller is closest to P4), while in Melee, the winner is the player with the lowest port number (whose controller is closest to P1). As multiple players must be KO'd on the same frame for this to occur and the games run at 60 frames per second, the chances of this occurring are slim and any advantage given to certain player numbers compared to others is decidedly negligible, especially since Sudden Death is never used in competitive play. In Brawl, tests have shown that the winner of the match is decided at random. In SSB4, the winner is whoever dealt the most damage, with placement of the other players following suit.
In SSB4's Tourney mode, Sudden Death doesn't occur in Regular Tourneys, and can be disabled in player-made Tourneys as well. If a tie occurs when Sudden Death is disabled, victory is awarded to the player who dealt the most damage throughout the match.
In competitive play
In tournaments, should Sudden Death occur, it is ignored, with the winner being decided by other factors. This is because of three main reasons:
As a result of being too imbalanced to be used to resolve ties, should Sudden Death occur in a tournament, the winner is declared by some other criteria. In the most common case of the clock expiring while both players are tied in remaining stocks, the player with the lower damage percentage wins. In the rare event of both players having equal stock and damage, or if both players are simultaneously KO'd (on the same frame) on their last stock, the winner is determined through a one-stock rematch with the same characters on the same stage. If both players are KO'd at the same time due to a sacrificial KO, tournaments often have a rule that declares the initiator the winner, instead of holding a one-stock rematch.