This might be considered along the same lines of "unintended use of code".

When I was around 13 I wrote a bash script that would automatically backup your browser data to an FTP server of your choosing. This included your the passwords database, and I made it work with just about every browser possible.

I released the script on some IT forum (I forget which) and it was a big hit for some reason, but then someone re-released it on hackforums and told people to hardcode the server credentials instead of using a settings file. Tricking someone to download a tiff, exe, whatever would upload their password databases from all their browsers in a matter of seconds.

It got quite a bit of attention (many thousands of views) and I've always felt bad about the amount of credentials that were possibly stolen.