Throwaway account. My company created an analytics product around the ability to track which sites your visitors have visited. It used a different and (at the time) more reliable technique.

About a year after the product launch we were contacted by a powerful washington based lobby group and they wanted to chat. They felt it violated a site visitor's "reasonable expectation of privacy". I agreed. So we pulled the feature and dodged a bullet as this "browser bug" hit the mainstream press a few months later. The feature wasn't a major part of our product's value prop, few of our customers used it and none missed it.

So if you're thinking about basing a startup on this, don't. You will get a call very quickly from organizations much larger than you are asking awkward questions.