It's been a long time since this incident occurred, but from what I remember, Safari had/has a (non-default?) setting that blocked third party cookies, but allowed them upon user interaction. At some point Google added +1 buttons to their ads, and some aspect of how they implemented that tricked Safari into thinking the user interacted with the ad even when they didn't and allowed cookies to be sent. After this was pointed out Google updated the +1 buttons to no longer trigger this behavior in Safari. (Correct me if I'm wrong; this all happened years ago so I may be misremembering.)