Several years ago I signed up for a free tier account with AWS to play with . But I also didn't know what I was going to do with it - I didn't create any resources. Got distracted, a few weeks went by, they sent me an invoice! It wasn't for much, but it was not $0. I was very confused (and never figured out why I was charged). Fortunately I was able to reach customer service and get the costs dropped, and I promptly closed my account so that I didn't accidentally (through no action of my own) accumulate further charges!
I'd like to see "get charged more than expected" or "think you were on the free tier but somehow you weren't" on this purity test.
EDIT: OK either "free tier" was added just now or I fat-fingered "free" when I searched for it earlier :)