I had an AWS account I wasn't using, but I didn't want to get surprised by any bills, so I set an alert to let me know when the bill was more than $0.
Sometime later the alert went off, so I anxiously jumped into AWS to see what kind of disaster was happening, and found that I got billed a few cents... for the alert itself.
I closed it altogether after that. It's better for my sanity to just use a new email address and sign up again than to worry about surprise bills.
EDIT: It's entirely possible that I set it up wrong, or set up the wrong thing, but in any case an alert was all I was trying to do, and as far as I could tell that's what happened. I must admit I rage-quit rather than investigate too deeply.