I recently bumped into a problem with Google Calendar. It wouldn't create new entries, no matter how I tried (direct clicking on timesheet, through the Create button, etc.). It worked perfectly in other browsers.

So I checked Firebug, and saw an XHR entry that was red and loaded very fast (4 ms): https://www.google.com/calendar/static/41160d4e4ae6c0f08ffb15dbc5124b31calendarjs_eventformcompiled__en.js

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