Example backtrace log
If you are interested in determining how cookies were set without human interaction, I/O Labs records the full backtrace so you can get a better understanding of how it happened. This log shows a domain (lose34pounds.com) cookie-stuffing users by presenting them with an eBay affiliate link, and then having a JavaScript function that "clicks" the link 4 seconds after the page loads.
View backtrace log for lose34pounds.com
You can see from the backtrace that visitors are forced to click an eBayListing.php URL 4 seconds after the page loads. This "click" was initiated by a script embedded within the page source. The backtrace will even tell you that the click happened within a function called myTimer, that starts on line 25 of the page source.
Looking at the page source, you can see that the I/O Labs backtrace is exactly correct:
