So at the old job, i needed support for an issue relating to Amazon S3. We used a third party Python plugin for sending files to our buckets, but had some pretty severe performance issues when trying a 2-way sync.
Naturally, I sought help on StackOverflow, and was asked to share my config. Without much thought, I pasted the config file.
Next comment made me aware that our API id and key was listed in this config (pretty rediculous to keep such private info in the same file as configuration, but oh well).
I edited my question and removed the keys, and did not think about the fact that revisions are stored.

Two weeks later, my boss asks me if I know why the Amazon bill is for 25.000$ when it used to be <100$ 😳

I've never been so scared in my life. Luckily, Amazon was nice enough to waive the entire fee, and I leaned a little about protecting vital information