We got lucky and the following Cucumber scenario caught the fact we weren't allowing a necessary parameter (fandom_tags_to_remove) it doesn't explicitly test:
Let's extend the tests to explicitly check that a tag set owner can add and remove each type of tag – fandom, character, relationship, freeform, warning, rating, and category – especially since warning, rating, and category work a little differently than the others.
Also, there was a scenario that hadn't been written yet:
Turns out we need that because we almost broke it /o\
How to test:
No manual testing.