In my experience, the best way to report such a error is to take a screenshot of it (of the character with the Guild Tabard equipped, in this case), then use either the keybind <F1> Help or enter /bug in the text entry field of the Chat UI. Choose the appropriate Feedback categories, describe the error, and checkmark the box to attach the screenshot, before you use the Submit button.When you have on an outfit made from an outfitting station, and put on a guild tabard, the tabard shows over the chest armor you have on but it also turns your chest/neck completely black.