Open a page with comments to an entry. Use the top 'post a comment' link. With the quick comments feature on, a box appears in the page. Enter some text, check the 'Check spelling and preview', then click the 'post comment' button. On the preview page, use the back button in the browser, then click the 'post comment' button again. You'll end up at an error page.
(Workaround: click on the 'post a comment' link again, and your comment box will resize a bit, and then you can click post comment again.)
Can anyone confirm that this isn't just me?
(Discovered while posting a comment on an lj_backend post while lj was acting up.)