I discovered that Tinder would return distances between users with extremely high precision. This is a problem because it allows you to deduce the exact location of a Tinder user by measuring their distance to three known points (this is called trilateration).
I made a demo application to geolocate Tinder users in order to demonstrate to Tinder how serious of an issue this was. They fixed the problem, and afterwards I disclosed it publicly. I got a bit of press for it too!