If you are using WebScarab and something on the page keeps breaking then you may be having an issue with WebScarab's "Reveal hidden fields in HTML Pages". Try turning that off and repeating the action. I've seen this be an issue a few different times with JavaScript and AJAX.
Proxy -> Miscellaneous
The reason this issue occurs is because, when reveal hidden fields is enabled, WebScarab will rewrite some of the page. I imagine WebScarab is getting confused with some content in JavaScript or AJAX and thats whats causing the problem. The reveal hidden fields feature actually works quite well, I've only seen this issue a few times.
Rogan, next time I see it I'll send you a bug report: :) I know, just commenting on the issue without a bug report doesn't help you fix the problem.
-Michael Coates