I recently noticed this glitch where if you go to a category that you cannot post in (such as a rules category) and you quote something, it sets the category to that category that you quoted from. Here is an example from the Stateview Forums: where I quoted one random word:
I can reproduce this issue on a test site, so it’s a Discourse-wide issue, but when this happens you’re not allowed to post the topic unless you change the category to one you have permission to create in.
Ideally we should update this to fallback to the default category or no category selected.