By using an item that will disappear aftere use, in this case giving Cider to locals of the Seer's Village pub, and by quickly selecting the item before it disappears, an item is still selected, it is called 'Null'. You cannot see any items selected, but if you put your mouse over another item, it shows 'null-->item'
BTW printscreen does not capture the runescape mouse, so you can't see it.

"Null"

'In fact, "null" was just the default setting for all names. Basically, it meant that the information on the player hadn't loaded yet and, since it didn't yet know the name, the computer just assigned the value "null". This occurrence was almost completely removed in the "Game engine upgraded!" update on 16 May 2006. '

Awesome! I just found a way of re-accessing Null! GO ME!

There was a glitch a few years ago where an item that looked like a dwarf's head was called 'null'; it was tradeable and poeple in world 1 actually traded santa hats/masks etc for it, idk what happened to it. And I noticed this today too.

Your essentially correct, however its a bit different.
In programming anything that does not have a value is refered to as null.
Now this glitch is caused by the Client sending the item id and the server searching for it. The server goes through a process of


This is used for everything related to Runescape, everything is give a uniqe identification in that if the server is not found it is refered to as null, which is why if you click on a player who logged out as you clicked you will get null.

Yeah, I program, so I know what the null value is. It's just a neat little glitch though.