I may have found a bug...or I am misinterpreting Pathfinder 1st Ed. rules?
+n enhancement bonuses on shields can be added to their armor effectiveness or weapon effectiveness.
I _think _one shouldn't be able to add a weapon special ability to a shield unless at least +1 has been added to the weapon effectiveness (adding to armor effectiveness shouldn't count)
I used Customize button to first add +1 armor enhancement to darkwood shield and then was able to Customize to add menacing weapon special ability
Not sure if this is code or data. I haven't tested with other weapon special abilities besides menacing.