When someone replies to a message that started with a mention it displays the HTML as part of that reply.
For example, note the <a class="mention" href="/u/simon_tomes">@simon_tomes</a> part of the reply.
I’d expect it to just display @simon_tomes and then the rest of the original message.
Thanks for the report Simon! Our code wizards are on it 
Excellent, thanks! Go Code Wizards!
Hi, yes we are focused on shipping good support for threads and will improve this after, it’s 100% on our roadmap.
In the chat, if you mention someone, have at least one message in between, and then have someone (or yourself) reply to the message with the mention, the reply will show something like <a class="mention" href="/u/firepup650">@Firepup650</a> instead of the mention. Including < and/or > in your message will become < and >. This seems to be a recent change, and seems a bit unintentional, because if your message is just a mention, it works fine, but any other text in the message will cause it to become HTML in replies.
I think this is the same as this open bug topic @Firepup650, so I’ve slipped your post over here to keep the reports grouped together. 
This does seem to apply even if the mention isn’t at the beginning of the message, and it applies to group mentions as well.
Thanks for the reports, the issue will be fixed by this commit:
