As admin for the Google Analytics Test project, I received detailed and very helpful email yesterday from a participant stating that he was encountering an error on the setup page of the site while using Firefox 16 as his browser.
Once I duplicated this error in my own installation of Firefox 16, I logged into GA, drilled down to Firefox 16 in the Browser & OS report, and saw that the Conversion Rate for Goal 1 (beginning the test) was only 2.90% for the previous week. Goal 1 Conversion Rates for earlier versions of Firefox, and for other browsers, were much closer to the site average of 39.64%.
I also saw the number Firefox 16 visitors likely impacted by this error in the previous week: 69.
Fortunately, the outstanding Firefox maintenance team at Mozilla was able to provide a quick fix for www.googleanalyticstest.com. (The problem: incompatibility of Firefox 16 with an old version of jQuery that I was still running.) And thanks to Google Analytics, and to the participant’s email, I had been able to isolate the error to a specific browser version and to determine how many visitors had been affected.