Tomayto, Tomahto 
I had a user complain to me that the undo function is broken and I passed on his findings here in a reproducible way.
Gmail, Basecamp and other WYSIWYG tools allow users to undo formatting changes. In Discourse’s case the formatting is simply the Markdown markup. Since shortcuts/buttons for the markup are available, those functions should be undo-able. StackEdit, for example, handles this gracefully. Not doing so subverts a reasonable user expectation.