[dredmorbius]

Two-pane is useful as it highlights precisely where you're fouling up Markdown in realtime. On far too many platforms (including HN) I have to go through multiple submit-edit cycles to try to fix buggy markup.

G+ is horrible at this as it parses double and single spaces differently (a double space is "<space><nbspace>").

On HN, it's the failure to recognize underbars. Since I write _italic_ and bold, I keep leaving underbars all over my comments. I'll notice and fix some of them (just nearly did it there with "some"), but often don't run across the issue until after the re-edit window has closed.

Reddit's RES double-pane editor is among the best I've encountered especially in full-screen view, screenshot:

http://i.imgur.com/ySsncbq.png