This may apply to photos and videos, not sure.
Creating a category and including a comma in it (eg. "Music - Pop, Pop Rock") and then uploading a sound in it:
- the activity will not be shown in the Outline block or the Spy;
- The category will not be shown on the categories page. It will be listed in the "User Categories" block though, as two categories, as (eg. "Music - Pop" and also "Pop Rock").
Similar to tags, the comma is a separator. No biggie. Just wish I had known - an hour of my life I will never get back. If I hadn't removed the "user categories" block I'd have noticed the category split right away.