I thought I had shut it down because I clicked some button that looked like it was the turn-off button. I let it go on for a few months, only to discover that I had been charged $1,500.
The one thing that really pissed me off was how easy it was to set up vs how hard it was to take it back down. I can't remember the details, but basically you cannot simply turn off an RDS instance only in the UI (even though you can turn it on in the UI). You have to install the SDK and perform some (seemingly complex) commands.
I tried explaining that I was a beginner, and that I made this mistake by accident, and that they could easily see that I had not actually used this instance at all or even put any data into the DB. But they wanted this huge list of things from me in order to refund it, like a super in-depth explanation of how it happened. Really shitty experience overall. So I canceled my AWS account and likely won't go back until I have a job that pays me to learn and use it.