Knife bug

The Knife bug was a glitch that occurred in 2002 wherein a player was able to equip multiple knives and gain an enormous attack bonus. This glitch would be caused by dropping a weapon, but opening the right-click menu for it, and having a knife in the next inventory slot. After the weapon was dropped, the knife would take the place of the weapon in the inventory and clicking the "wield" option on the menu would cause it to become equipped. Jagex took action against any abusers by banning those who used them for Player Killing and temporarily banning those who used them for melee training.

It occurred because knives were originally intended to be equipable. The option (and thus assigned slot) was removed, but the bonuses remained.