The Javascript editor chokes on the following characters:
*
, $
, `\
, [
, ]
, ^
, '
, "
, {
, }
, \
I'm sure there are others, but those are the ones right there on the keyboard for me to test. Some of those are obvious and not bugs ('
and "
, for example, are used to provided title text). I could test the server-side Markdown processor, but that'd be difficult and unpleasant.
The original wmd editor doesn't appear to have these bugs, which implies that it's something in the showdown engine. I would imagine the server-side version was designed to mimic the preview editor as much as possible, which would explain why the bugs persist in the rendered posts. That's just a guess, though.