I've had a few issues seemingly at random when quoting another poster.

I'd hit the 'Quote' button, and the next page would load as normal with their post in the BBC code (or whatever). I'd write my reply/response and hit 'Post' This is where the random issue pops up.

Instead of attempting to post it as a comment within the thread, it would try to post it as a new topic. It would give me the error that I needed to enter a topic (which was never an option when the quoting page was up).

Ultimately when I ran into that issue, I'd just copy and past the entire post and do a quick reply instead.

Just FYI