“Final Fantasy VII” has a system in place where both damage and HP of any given character cannot exceed 9,999. That means, a character can have 9,999 HP, and deal 9,999 damage per attack, but nothing higher.
This does not mean that damage is limited to 9,999 per turn, as Materia such as Double Cut can have a character deal 19,998 or even 39,996 damage in one turn due to the multiple strikes that it allows. Other Materia can have similar effects, however Materia is not the only way to deal massive amounts of damage in a single turn.
“Final Fantasy VII” has somewhat of an Easter egg, where if any given character's HP is 7,777 exactly, that character's name will flash with rainbow colors and become uncontrollable in the next battle that is entered. Because it is so hard to get a character's HP exactly to 7,777, this tactic is usually achieved through pure luck, or someone who was intent on getting the Lucky 7s effect.
What happens is that the character effected will go into a frenzy and attack every enemy on the field for a total of 63 times, each time dealing 7,777 damage, regardless of weapon, regular attack damage, or enemy defense. After the fight that the Lucky 7s was active in, the character afflicted will have his or her HP reduced to 1, so that Lucky 7s will not active successively. If used on a difficult boss, that means it could take a total of 489,951 damage at the very beginning of the battle. The damage is considered to be done in one turn and therefor cannot be interrupted and no other character can act until the animation is done and the damage has been dealt.
Most bosses do not have anywhere near that amount of HP, so it is almost a guaranteed 1-shot kill, however if used on a super-boss such as Emerald Weapon, it will only take about half of his health, as he stands at a total of 1,000,000 HP. Though, it is a very effective way to reduce his health by half at the start of the battle.