Made a similar mistake like this once. While just playing around to see what's possible I upload some data to the AWS algo that will recommended products to your users based on everyone's previous purchases.

I uploaded a small xls with uid and prodid columns and then kind of forgot about it.

A few months later I get a note from bank saying your account is overdrawn. The account is only used for freelancing work which I wasn't doing at the time, so I never checked that account.

Looks like AWS was charging me over 1K / month while the algo continuously worked on that bit of data that was uploaded one time. They charged until there was no money left.

That was about 5K in weekend earnings gone. Several months worth of salary in my main job. That was a lot of money for me.

Few times I've felt so horrible.

I worked in a billing department, and learned to be healthily paranoid about such things. I want to regularly check what I'm billed for. I of course check all my bank accounts' balances at least once a day. All billing emails are marked important in my inbox, and I actually open them.

And of course I give every online service a separate virtual credit card (via privacy dot com, but your bank may issue them directly) with a spend limit set pretty close to the expected usage.