I never have left something running like that, but someone else in the org had made a service that would spin a t1.micro for end users to experiment with one of our saas offerings. Iirc the instance would run for 24 hours maybe, and then shut down and delete itself. It didn't delete the ebs volume, though.

So about 6-9 months after I left I get an email, please delete these, I'm too busy to use the UI to delete these 8000 ebs volumes. I don't know how much that cost, even for the final bill month, but it took me an hour and a half to write and test the script, hour and a half to run it, and an hour for my time to get reqs and prepare an invoice.

I've managed AWS for corps that Have spends in the millions of dollars per year (and knowing how split brain org charts are, possibly millions per month) on AWS and other platforms. Actually I know it was in the millions per month range, two transcoder services were costing $250k/m. Obnoxious.