When I change the category a topic is in (via the edit topic title thingo) I am finding that I cannot also tag. If I try, I get the message “this topic has tags that aren’t allowed” - when this is not the case.
My workaround is to save the category change, and then re-open the edit topic title thingo, and then add the tags. It works just fine then. Neither category limit tags in any way.
This is very similar (or even the same) as:
Can you reproduce it? Happy to do a video if you like.
Hi, yes I can imagine how it happens, will have a look at this by end of next week.
Hmm, I just changed this from bug to support and added the “feedback” tag at the same time, in the same edit. No repro. Can you give me more specifics please?
Worked it out:
It only happens when the tag doesn’t yet exist. I can change categories and tag with an existing tag, but not with a new tag.
A bit similar to this, eh?
Thanks, I have edited the title to be more specific! 
I’ll be looking at this @nathank. It’s a tricky one.
Actually, not so tricky. I made a fix just now.
https://github.com/discourse/discourse/commit/29e733832d7f95669310a662594ca53b07938f38