When using the ApiSandbox with options such as json/query/prop=categories, you get inputs that look like this:
titles [ ] A list of titles to work on
pageids [ ] A list of page IDs to work on
revids [ ] A list of revision IDs to work on
These three instructions don't mention what the separator is! I think it's most natural to assume that the values are comma-separated, but they're surely not. The default documentation should be updated to say something like... "A list of titles to work on, separated by '|'" or something.