You were using the hash character. FT interprets that as a link to a poster's forum name and makes it into a URL.

This relatively new behaviour trips up many users, especially given that FT has a prior (and long established) use for the hash character. It's known as the "number sign" or "pound sign" in N America and appears in front of the post numbering used within every thread. So many people wanting to reference "post 2 above" include the symbol -- and the system generates a link to an unhelpful list of posts where the same "hash tag" has been used.

This has been raised before (e.g. here in the thread this has now been moved to) but not, as far as I know, resolved.