As a Stack Overflow member, is it possible to participate in fixing bugs with the client code for Stack Overflow (or sister Stack Exchange sites)? I've only been using these sites for 4 months, but it never cooperates with certain syntax. It would also be nice if it would assume line breaks when there is a line break in the editor. I also wish it had a feature to be able to put code into a set of tags (like a wiki).
Regarding line breaks, it is covered here: Should the markdown renderer treat a single line break as <br>?
Regarding supporting our editor, both wmd and showdown live here. Balpha is in the midst of a refactor at the moment, we should upload latest in the next few weeks and have 100% parity with the public version. At that point you can clone and make pull requests for fixes.
If you submitted patches to the Google Prettify project that were approved, then you could indirectly improve the syntax highlighting found on StackOverflow if SO incorporated them downstream.
However, my gut feeling is you should take a look at the Prettify Hints supported, which can vastly improve the syntax highlighting in various posts.
If that fails, you can submit a question on meta with a link to a question or answer and a picture detailing the bug you may have found. Or you can go to the horses mouth (the project site) and submit the bug reports there.
You can contribute here by in a very positive way by describing what behaviors you think are bugs and filing them as bug questions or clearly defining desired changes in behavior and filing them as feature-request questions.
One of the hardest parts of the software process for a stable and evolved software product is getting consensus that what you think is a bug really is a bug or that what you think should be a feature will be widely accepted as an improvement. This is because a lot of people have already thought long and hard about the software and there isn't much "low hanging fruit" to harvest.
Even in areas where you can't contribute code, history shows that some bug reports do get fixed and some feature request do get implemented. Good bug reports take insight and good feature requests require creativity and consensus is hard work. Sometimes making the code changes is the easiest part of the process!