Null

  is a reserved constant used in and similar programming languages that is used to represent a void reference. In simpler terms, it is a value used to represent that a reference variable has no set value. In RuneScape, it usually appears in string values either due to a variable not being assigned in time, usually due to lag, or as a result of a glitch.

It first made its appearance in game as a player name some people would see from time to time in RuneScape Classic. This can occasionally appear when moving between areas like going up and down stairs. This usually occurred when players with dial-up, or similar low speed, internet logged in and tried to type something.

Null items
An item also can be called "null". It means the same at the top of the page—the information of the object didn't load. Strangely, the "null" object's name never changed.

Make null
Sometimes, when making items, "null" is displayed for a split second in the interface. This tends to happen more often while smelting. Also, null can be made by dropping an object, then right clicking the object again before it is removed from your inventory, click "Use" and right click on another object.

Logging in
Sometimes, immediately after logging in, if a player talks, or the game makes the player say something, the player's name will appear as 'null' in the chat box.