Back when I used to freelance, somebody once paid me a scant fee to build a pretty cool online tool for music teachers, basically an interactive piano. I checked in on the website a few years afterwards and found that they had wrapped it into an app and been selling it for $5/install ever since I made it for them. Probably my fault for not licensing the software I wrote for them (I was basically a kid at the time) but it still bothers me.