javascript - Why have "while(1);" in XmlHttpRequest response?

I'm
playing firebug with Google Calendar. I happens to find that some XHR request has
response like
below:

while(1);[['us','bW9yZ2FuLmNoZW5nbW9AZ21haWwuY29t
            20090320/20090904
            63378122163']]

It
looks like a JSON with prefix dead-loop javascript
statements.

I'm not sure why Google Calendar has
such kind of XHR response. Is there any known AJAX practice about
this?