We've come to rename Markdown to Letdown in the C++ chat room because it lets you down so often. I've now just found a pattern.
It seems markdown fails for multi-line messages. That is, this
Letdown can't cope with multi-line comments.
fails to display
code marked as code, while this works
Now, I think it would be Ok if you said chat messages shouldn't be multi-line. (Although I'd be a bit annoyed, because I like them.) But if so, please don't render them as multi-line (as is the case with comments), so we aren't tempted to use them.