I recently lost $300 because of an API key leak. It started with a surprise $200 charge from Google Cloud, and when I looked into it, I found another $100 charge from the day before. Both were for Gemini API usage that I never intentionally set up.

After digging, I discovered the issue: I had hard-coded an API key in a script that was part of a feature I ended up deprecating. The file was only in the codebase for two days, but that was enough for the key to leak. Google actually sent me alerts about unusual activity, but I missed them because they went to a less-frequently-checked email account.

Here’s what I learned:

Never hardcode API keys - Use environment variables or a .env file, even for temporary code.

Set up billing alerts - Google Cloud (and other providers) let you set up alerts for unexpected charges.

Check all linked emails - Don’t ignore notifications, even if they’re sent to secondary accounts.

Don’t rely solely on GitHub’s secret scanning - It’s useful, but renaming variables can bypass it.

This happened while I was experimenting with "vibe coding" (letting AI generate code quickly), but I realized too late that human oversight is still crucial, especially for security.

Hope this helps someone avoid the same costly mistake!

TL;DR: Hard-coded an API key in a deprecated script, key leaked, and I got charged $300. Always use environment variables and set up billing alerts!