Finding all nine items hidden items

In this post, the developer confirmed that they designed the inventory screen to have space for 9 items before they completed the game, which actually only holds 6 or 7 items.

From that post, the developer says:

Yep, you're right about the 7th item thing-- for at least one frame of computation, you have both halves of the combo in your inventory. Then when the game detects that you have both of them, it removes them and replaces them with the completed combo. So that extra slot needs to be there to support it. That said, we designed the UI before we knew how many inventory items we'd have in the game-- we just knew we'd need to not have more than 9 ;)