It's definitely a worry, but to their credit AWS at least are pretty nice about accidentally racking up a big bill. Last month I forgot to shut down a GPU instance I'd been experimenting with on my personal account and got hit with $150 I hadn't been expecting, but on explaining the situation to support they cancelled the bill from the previous month and applied a credit to my account to cover the time it had been active this month. I'm going to guess that's a one time thing, but the gesture was very much appreciated.