Comment by geocar

I wrote about something like this previously[1]

Ads often have something like this attached to the end:

    load("https://adserver/track.gif?{stuff}&_=" + Math.random())

My ad server collected multiple tracking pixels -- for various events and after five pixels I could fingerprint the browser (Firefox, Chrome, MSIE, etc) and identify someone fiddling with the user agent string, or using a proxy server to mask this information.

[1]: http://geocar.sdf1.org/browser-verification.html