I'm sorry but anyone that sees a multi million or billion dollar bill on an account that does nowhere near that should not be scared. It's obviously a mistake. Stories like this have happened with banks in my country. Check your account and you have billions in there. Guess what happened to those that withdrew money? The judge told them any reasonable person would know this is a bug. Had to give it back. Same thing here, any reasonable person doesn't get scared.

The problem with an anomalous AWS bill is that I don't know if it's because of a bug on their end or because of a goof on my end, eg did I accidentally leave on some giant compute for a month or something. With a billion in my bank account I at least know that I definitely didn't put it in there.

If I experience a bug when transferring money (switching a digit or some such) they’ll shrug their shoulders and say there is nothing they can do. No amount of proof will push them to increment and decrement some integers over there.

They transfer money to me. Their mistake: there is nothing I can do. Rules for me but not for thee?

I’d be concerned as it throws into doubt their entire accounting mechanism