Not who you asked, but my mistake was transferring an S3 bucket full of unused old customer web assets to glacier, we were paying a lot to host them each month, and weren't using them anymore.

I set the lifecycle rule on all objects in the bucket, for as soon as possible (24 hours).

About 2 days later first thing in the morning I get a bunch of frantic messages from my manager that whatever script I was running, please stop it, before I'd even done anything for the day.

The lifecycle rule had taken effect near the end of the previous day, and he was just getting all the billing alerts from overnight, it was all done.

I read about glacier pricing, but didn't realize there was a lifecycle transfer fee per 1000 objects (I forget the exact price, maybe $0.05 per 1000 objects). That section was a lot further down the pricing page.

The bucket contained over 700 million small files.

I'd just blown $42,000.

That was over a month's AWS budget for us, in the end AWS gave us 10% back.

On the plus side, I didn't get in too much trouble, and given we'd break even in 4 years on S3 costs, upper management was gracious enough to see it as an unplanned investment.

TLDR: My company spent 42k for me to learn to read to the bottom of every AWS pricing page.