I've seen some streamer's do this, they'll have a unit with an item already on it, and build the item they need using the item on that unit but the completed item is dropped out of the unit instead of having to sell the unit.
Was it the ones that add on types?
Those must be trait items, built with Spatula. The rule is simple:
Example: If you build 
, which gives the trait Blademaster to its holder, on 
, it will pop off of him, because Draven is originally a blademaster anyway, which makes sense.
Example: If you make 
(Hextech / Gunslinger) a Blademaster with , then you build another on her, it won't pop off of her, even though, technically by that time, she is considered a blademaster herself, because of the 1st . And the reason is very simple: Jinx is NOT a blademaster originally. So, be careful in those situations, otherwise you'll get stuck with a Jinx with two on her.
They are adding an item next patch that will let you.
