In the Audience section, the workflow for adding and removing tags from multiple contacts is incredibly unclear and easy to mess up. When you select multiple contacts and click "Edit", then click on either function, the edit panel in both cases shows a list of ALL tags that are present on at least one of the selected contacts. It looks like you're adding or removing all of the shown tags from everyone, but that's not the case. Worse yet, when removing a tag there is zero indication that what you ACTUALLY have to do is X out the tag you want to remove, and leave the others in place when you click Apply.
(Oh, also, the changes appear to take place before you hit Apply for extra fun and giggles if you make a mistake.)
This is simply crazy. I had to go back and repair 25 contacts' tags manually because I misunderstood the way this works.
Please change this to work like literally every other CRM ever invented: Add Tag should show a dropdown of tags and let you choose one or more you want to ADD. Remove tag should show a dropdown that lets you choose one or more you want to REMOVE. NOTHING should actually take effect until you hit Apply.