"Using" an armor item when it's in the shield slot makes a copy of the item when in creative mode.

Steps to reproduce:
1) ensure you're in creative mode
2) put a diamond helmet in the shield slot
3) right click - you hear the "putting on armor" sound
4) go back to inventory. You now have two helmets. one in the shield slot and one on your head.

This does not happen in survival. In creative, it will work with any armor item.