I signed up for emusic.com, and incorrectly interpreted this:
to mean that I wouldn't be charged anything unless I downloaded more than 45 songs, at which point I'd be charged 50 cents per song.
To avoid surprises, I used a shopsafe credit card number, and set the spending limit to $10.
It turns out that I misinterpreted the charge structure (my fault), and misinterpreted how ShopSafe works (not my fault), because an 11.99 charge appeared on my credit card. I spent time calling BofA to figure out why the charge went through, but never got an answer. It's not worth disputing.
Anyone know why the charge wasn't rejected?