Dragon knife glitch

The Dragon knife glitch (also known as the Super Kill glitch and PVP Kill glitch by some players) was an extremely severe glitch that was first found soon after the release of the Dragon knife

When used with powerful weapons or weapons with powerful special attacks (i.e. Dragon claws (most infamous weapon used in the glitch), Armadyl godsword and other godswords, Dragon dagger, Dragon sword, etc.), the glitch nearly always assured an instant kill.

In a matter of only 3 days (the amount of time that the glitch lasted from discovery), billions of coins in items were lost, and one case even reported a Blue partyhat lost, worth over 900 million coins (subject to change).

Mechanics
See article: Dragon knife

The special of the Dragon knife, Enrage, was the main factor in the glitch. When enabled, 10% of the special bar was drained, and the player's attack speed was lowered to 5 (including on Rapid). However, whenever the player would attack, they would throw three knives at once, with each knife having 15% increased accuracy and 10% increased strength. This happens 5 times every enabling of the attack, giving 50 attacks per full special bar.

However, when enabled, players could switch to another weapon just as the knife attack occurs, "loading" the attack. The attack with the second weapon (the one switched to) would not occur, assuring safety. "Loading" the attack is different as you can do this in a non-PVP area, to players you can't fight (due to level difference), and from distances where you could attack a player with knives who was in a PVP area while you weren't (you would stand still, anyway). This would only need to be done once, with no further "loading" needed (unless a player ran out of special attack energy, switched weapons, logged out, etc.). Because of the mechanics of the knife special, the player could hit 3 times with increased strength and accuracy with the weapon of their choice 5 times, with only draining 10% of the special bar. This means that a player can execute this type of attack 50 times per bar.

However, this glitch also worked with special attacks of other weapons. A player could "load" the attack and also use the special attack of the other weapon, essentially combining the two weapons' special attacks with only the drain of the Dragon knife's special. For example, if loaded with Dragon claws, players could essentially do a 12-hit dragon claw special attack (3 special attacks* 4 attacks per special), while also having the increased power of the Dragon knife's special attack and having 5 attacks in this fashion per 10% of the special bar. This meant that the player could do a triple Dragon Claw special attack 50 times per bar. Also, if the knives were poisoned, poison damage would automatically occur per special, meaning that 3 hits of poison could hit along with the claws, adding to the damage.

Impact
As one can see (by the Dragon claw example in the Mechanics section- see above), the power of the glitch is extremely powerful. If using Dragon claws in the glitch, one is nearly always assured an instant kill. Because the bar allows 5 attacks of this type per drain (10%), one could essentially obtain 50 instant kills per attack bar. The glitch, however, remained unknown by most of the community, allowing it to thrive.

Even the strongest of armors could not stand the power of the glitch. In a test, a player with full Torva armor, runescape:Saradomin brews, Extreme defence, and the Staff of light special could not stand up to a player with 90+ melee stats, Turmoil, Overload, and Dragon claws and was instantly killed by the special attack.

The glitch proved to be catastrophic in losses, with approximately 25 billion coins lost in items during the glitch lasted (only 3 days). One case included the loss of a Blue partyhat, valued at 900 million coins, though subject to change.

Jagex's Response
After Jagex recieved info on the glitch, they immediately shut down RuneScape's servers for a fix. Then they immediately banned hundreds of abusers of the glitch. However, they could not return any items, as many players lied about losing items, even though they were not affected.

"We were shocked when we acknowledged the damage caused by the glitched. We regret the losses of many of the players, including those with items worth several million of coins. Hundreds of abuses have been banned, and they will have no chance of having their accounts returned. However, we cannot return the items lost, as many players weren't even affected by the glitch. Please acknowledge our apologies, and we hope that you are able to regain your valuables which some of you have lost over the past couple of days."  -Official statement by Jagex