Unfortunately, this is a "feature" in Chrome, called "Reflective XSS Protection". It seems this has been in Chrome since version 4:
In Google Chrome 4, we've added an experimental feature to help mitigate one form of XSS, reflective XSS. The XSS filter checks whether a script that's about to run on a web page is also present in the request that fetched that web page. If the script is present in the request, that's a strong indication that the web server might have been tricked into reflecting the script.
We have only recently started receiving these reports; however, the "jQuery isn't working" warning isn't very old yet either, so it's possible that people previously just hadn't noticed that jQuery was blocked for the duration of one page.
Why Chrome wants to disallow people from XSSing themselves (this happens as a reaction to a same-origin POST request -- and it's data the browser itself submitted), I don't know -- in my book, this behaviour doesn't make any sense.
Sad as it is, there's not much we can do about it – Fortunately, it's pretty rare that a post submission contains JavaScript code that's identical to some code in the resulting page, and the workaround is easy (just reload the page).