There was definitely some failed startup that just published people's credit card purchases in a giant feed... maybe it was just an earlier iteration of Venmo? I've been Googling but can't find it.
Facebook had a feature that showed your friends purchases you made on your timeline. People were pissed that their surprise gifts were ruined.
Man unintended side effect, wow, I wonder how many other ones there were with things like this? Finding out someones sexual orientation when they order a sex toy?