At the time (2010), link shortening services were all the rage, but they were being used to obfuscate malware links. To fight this, we wanted to create a link shortening service that couldn't be used for malware. Instead of redirecting to a webpage, it broke the page apart into its core elements so you could see what it was about before you visited it. If the site had photos, we even put them into a nice looking image gallery that you could peruse.
We finished the project in 72 hours and got an honorable mention for the competition, but we didn't win. A month later, our free server hosting we were given for the competition was about to expire. I logged in to spin the whole thing down.
Poking around the DB a bit, I realized our little weekend project had thousands of links shortened and the traffic for the site was actually exceeding the limits of the server. Looking at the URLs, I realized they all had one thing in common; they were all porn sites. The simple inclusion of the image gallery had inadvertently created a porn scraping service. It was like your browser's reader view, but perfect for adult sites!
It was so obvious in retrospect, but sometimes in your rush to build an idea, you can be completely blind to its true nature. This time it was amusing, but the next time it could be dangerous, and the lesson always stuck with me.