Equipable non-equipment items
Here is an example of it being done purposely. I assume he accidentally triggered this by acquiring the Bone Plates while unequipping.
Equipable non-equipment items
Invisible items wouldn't allow for equipping non-equipable items since the server checks ItemClass. The actual bug here is that on unequip any item in the destination slot upon completion is…
[Exploit] Spell casts do not check if you actually have enough nano to cast
[Exploit] Equipping only checks ItemClass, not Slot
[Exploit] Using items does not check CanFlags
[Exploit] Accepting a team request does not require any prior invite