Sudden Death

From SmashWiki, the Super Smash Bros. wiki

Sudden Death occurs when a vs. mode match ends in a tie. All players are given 300% damage, as opposed to zero, making them easy to KO. This mode can be instantly accessed via the Special Melee mode "Super Sudden Death". After a certain amount of time, Bob-ombs will begin to fall from the sky, a feature which is absent from the Special Melee version. This is also present in special Brawl, under the name 300%.

Sudden Death almost never occurs in Stock mode (barring a timed Stock Match), though it is possible if all characters lose their last life at the exact same time (to the second). Certain forms of Kirbycide can easily create this situation in two player matches, and having all characters on a horizontal platform in Icicle Mountain can also do such. If all characters die at the same time in Sudden Death, the player with the highest player number will win (ie if Player 1 and Player 2 tie in Sudden Death, Player 1 will win). This gives a negligible, but technically true, advantage to players with lower player numbers.