So, today I was playing a ghoul bulwark and invested into the galvanic retributor (yes that all sounds like a terrible combo) and I found something interesting... the retributor doesn't go into the tinker slot like other tinkers, but applies directly to the shield and then says 'failed to apply tinker'.
This can be done an infinite number of times, and persists when the shield is taken and put back into the inventory.
HOWEVER, this only happened with one specific shield-- when I tried it with other shields the item worked just fine. Swapping back to the bugged shield would forcibly de-equip the galvinator (and apply it to the bugged shield again). So it's probably something to do with the shields egos (cosmic?).
https://gyazo.com/92dfd00772cc3dcd46f5a6e0ee5a53afHere's the shield in question... notice the stacking lightning damage on hit.